Major depression significantly affects a person's family and personal relationships, work or school life, sleeping and eating habits, and general health. Its impact on functioning and well-being has been compared to that of other chronic medical conditions such as diabetes.

A person having a major depressive episode usually exhibits a very low mood, which pervades all aspects of life, and an inability to experience pleasure in activities that were formerly enjoyed. Depressed people may be preoccupied with, or ruminate over, thoughts and feelings of worthlessness, inappropriate guilt or regret, helplessness, hopelessness, and self-hatred. In severe cases, depressed people may have symptoms of psychosis. These symptoms include delusions or, less commonly, hallucinations, usually unpleasant. Other symptoms of depression include poor concentration and memory (especially in those with melancholic or psychotic features), withdrawal from social situations and activities, reduced sex drive, irritability, and thoughts of death or suicide. Insomnia is common among the depressed. In the typical pattern, a person wakes very early and cannot get back to sleep. Hypersomnia, or oversleeping, can also happen. Some antidepressants may also cause insomnia due to their stimulating effect.

A depressed person may report multiple physical symptoms such as fatigue, headaches, or digestive problems; physical complaints are the most common presenting problem in developing countries, according to the World Health Organization's criteria for depression. Appetite often decreases, with resulting weight loss, although increased appetite and weight gain occasionally occur. Family and friends may notice that the person's behavior is either agitated or lethargic. Older depressed people may have cognitive symptoms of recent onset, such as forgetfulness, and a more noticeable slowing of movements. Depression often coexists with physical disorders common among the elderly, such as stroke, other cardiovascular diseases, Parkinson's disease, and chronic obstructive pulmonary disease.

Depressed children may often display an irritable mood rather than a depressed mood, and show varying symptoms depending on age and situation. Most lose interest in school and show a decline in academic performance. They may be described as clingy, demanding, dependent, or insecure. Diagnosis may be delayed or missed when symptoms are interpreted as normal moodiness.

To meet criteria for a major depressive episode, a patient must have 5 of these 9 symptoms nearly every day for at least 2 weeks.

1. Depressed or sad mood

2. Anhedonia (loss of interest in pleasurable activities)

3. Sleep disturbance (increased or decreased sleep)

4. Appetite disturbance (increased or decreased appetite) typically with weight change

5. Energy disturbance (increased or decreased energy/activity level), usually fatigue

6. Poor memory and/or concentration

7. Feelings of guilt or worthlessness

8. Psychomotor retardation or agitation (a change in mental and physical speed perceived by other people)

9. Thoughts of wishing you were dead; suicidal ideation or suicide attempts

Major depression frequently co-occurs with other psychiatric problems. The 1990–92 "National Comorbidity Survey" (US) reports that half of those with major depression also have lifetime anxiety and its associated disorders such as generalized anxiety disorder. Anxiety symptoms can have a major impact on the course of a depressive illness, with delayed recovery, increased risk of relapse, greater disability and increased suicide attempts. There are increased rates of alcohol and drug abuse and particularly dependence, and around a third of individuals diagnosed with ADHD develop comorbid depression. Post-traumatic stress disorder and depression often co-occur. Depression may also coexist with attention deficit hyperactivity disorder (ADHD), complicating the diagnosis and treatment of both. Depression is also frequently comorbid with alcohol abuse and personality disorders.

Depression and pain often co-occur. One or more pain symptoms are present in 65% of depressed patients, and anywhere from 5 to 85% of patients with pain will be suffering from depression, depending on the setting; there is a lower prevalence in general practice, and higher in specialty clinics. The diagnosis of depression is often delayed or missed, and the outcome can worsen if the depression is noticed but completely misunderstood.

Depression is also associated with a 1.5- to 2-fold increased risk of cardiovascular disease, independent of other known risk factors, and is itself linked directly or indirectly to risk factors such as smoking and obesity. People with major depression are less likely to follow medical recommendations for treating and preventing cardiovascular disorders, which further increases their risk of medical complications. In addition, cardiologists may not recognize underlying depression that complicates a cardiovascular problem under their care.

Minor depressive disorder is very similar to major depressive disorder in the symptoms present. Generally, a person's mood is affected by thoughts and feelings of being sad or down on themself or by a loss of interest in nearly all activities. People can experience ups and downs in their life everyday where an event, action, stress or many other factors can affect their feelings on that day. However, depression occurs when those feelings of sadness persist for longer than a few weeks.

A person is considered to have minor depressive disorder if they experience 2 to 4 depressive symptoms during a 2-week period. The Diagnostic and Statistical Manual of Mental Disorders lists the major depressive symptoms. Depressed mood most of the day and/or loss of interest or pleasure in normal activities must be experienced by the individual to be considered to have minor depressive disorder. Without either of these two symptoms, the disorder is not classified as minor depressive disorder. Other depressive symptoms include significant weight loss or weight gain without trying to diet (an increase/decrease in appetite can provide clues as well), insomnia or hypersomnia, psychomotor agitation or psychomotor retardation, fatigue or loss of energy, and feelings of worthlessness or excessive guilt.

All of these signs can compound on each other to create the last major symptom group of minor depressive disorder: thoughts of death, suicidal thoughts, plans to commit suicide, or a suicide attempt.

Minor depressive disorder differs from major depressive disorder in the number of symptoms present with 5 or more symptoms necessary for a diagnosis of major depressive disorder. Both disorders require either depressed mood or loss of interest or pleasure in normal activities to be one of the symptoms and the symptoms need to be present for two weeks or longer. Symptoms also must be present for the majority of the length of a day and present for a majority of the days in the two-week period. Diagnosis can only occur if the symptoms cause "clinically significant distress or impairment". Dysthymia consists of the same depressive symptoms, but its main differentiable feature is its longer-lasting nature as compared to minor depressive disorder. Dysthymia was replaced in the DSM-5 by persistent depressive disorder, which combined dysthymia with chronic major depressive disorder.

The exact changes in brain chemistry and function that cause either late life or earlier-onset depression are unknown. It is known, however, that brain changes can be triggered by the stresses of certain life events such as illness, childbirth, death of a loved one, life transitions (such as retirement), interpersonal conflicts, or social isolation. Risk factors for depression in elderly persons include a history of depression, chronic medical illness, female sex, being single or divorced, brain disease, alcohol abuse, use of certain medications, and stressful life events.

The forefront indication a depressive episode is manifesting is the sudden loss of energy or motivation in daily routines. When this occurs, it is not uncommon for individuals to seek medical attention with excessive worrying or anxiety that a more severe, physiological disease may be the underlying issue. However, without an actual disease present, this neurotic thinking often results in severe anxiety, sleep disturbance, and mood swings which may hinder social relationships. Individuals with endogenous depression may experience inconsistencies in symptom severity which is often the reason for delayed treatment. If left untreated, symptoms may progress to a major depressive episode.

Comorbid psychiatric disorders commonly go undetected in the treatment of depression. If left untreated, the symptoms of these disorders can interfere with both evaluation and treatment.

Anxiety disorders are one of the most common disorder types associated with treatment-resistant depression. The two disorders commonly co-exist, and have some similar symptoms. Some studies have shown that patients with both MDD and panic disorder are the most likely to be nonresponsive to treatment.

Substance abuse may also be a predictor of treatment-resistant depression. It may cause depressed patients to be noncompliant in their treatment, and the effects of certain substances can worsen the effects of depression.

Other psychiatric disorders that may predict treatment-resistant depression include personality disorders, obsessive compulsive disorder, and eating disorders.

Dysthymia characteristics include an extended period of depressed mood combined with at least two other symptoms which may include insomnia or hypersomnia, fatigue or low energy, eating changes (more or less), low self-esteem, or feelings of hopelessness. Poor concentration or difficulty making decisions are treated as another possible symptom. Mild degrees of dysthymia may result in people withdrawing from stress and avoiding opportunities for failure. In more severe cases of dysthymia, people may even withdraw from daily activities. They will usually find little pleasure in usual activities and pastimes. Diagnosis of dysthymia can be difficult because of the subtle nature of the symptoms and patients can often hide them in social situations, making it challenging for others to detect symptoms. Additionally, dysthymia often occurs at the same time as other psychological disorders, which adds a level of complexity in determining the presence of dysthymia, particularly because there is often an overlap in the symptoms of disorders. There is a high incidence of comorbid illness in those with dysthymia. Suicidal behavior is also a particular problem with persons with dysthymia. It is vital to look for signs of major depression, panic disorder, generalised anxiety disorder, alcohol and substance misuse and personality disorder.

Depression is a state of a low mood and aversion to activity that can affect a person's thoughts, behavior, feelings, and sense of well-being. A depressed mood is a normal temporary reaction to life events such as loss of a loved one. It is also a symptom of some physical diseases and a side effect of some drugs and medical treatments. Depressed mood is also a symptom of some mood disorders such as major depressive disorder or dysthymia.

People with a depressed mood may be notably sad, anxious, or empty; they may also feel notably hopeless, helpless, dejected, or worthless. Other symptoms expressed may include senses of guilt, irritability, or anger. Further feelings expressed by these individuals may include feeling ashamed or an expressed restlessness. These individuals may notably lose interest in activities that they once considered pleasurable to family and friends or otherwise experience either a loss of appetite or overeating. Experiencing problems concentrating, remembering general facts or details, otherwise making decisions or experiencing relationship difficulties may also be notable factors in these individuals' depression and may also lead to their attempting or actually dying by suicide.

Expressed insomnia, excessive sleeping, fatigue, and vocalizing general aches, pains, and digestive problems and a reduced energy may also be present in individuals experiencing depression.

Endogenous depression was initially considered valuable as a means of diagnostic differentiation with reactive depression. While the latter's onset could be attributed to adverse life events and treated with talk therapy, the former would indicate treatment with antidepressants. Indeed, this view of endogenous depression is at the root of the popular view that mood disorders are a reflection of a 'chemical imbalance' in the brain. More recent research has shown that the probability of an endogenous depression patient experiencing an adverse life event prior to a depressive episode is roughly the same as for a reactive depression patient and the efficacy of antidepressant therapy bears no statistical correlation with the patient's diagnostic classification along this axis.

Dysthymia, now known as persistent depressive disorder (PDD), is a mood disorder consisting of the same cognitive and physical problems as depression, with less severe but longer-lasting symptoms. The concept was coined by Robert Spitzer as a replacement for the term "depressive personality" in the late 1970s.

According to the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders, published in 1994 (DSM-IV), dysthymia is a serious state of chronic depression, which persists for at least two years (one year for children and adolescents). Dysthymia is less acute and severe than major depressive disorder. As dysthymia is a chronic disorder, sufferers may experience symptoms for many years before it is diagnosed, if diagnosis occurs at all. As a result, they may believe that depression is a part of their character, so they may not even discuss their symptoms with doctors, family members or friends.

Dysthymia often co-occurs with other mental disorders. A "double depression" is the occurrence of episodes of major depression in addition to dysthymia. Switching between periods of dysthymic moods and periods of hypomanic moods is indicative of cyclothymia, which is a mild variant of bipolar disorder.

In the DSM-5, dysthymia is replaced by persistent depressive disorder. This new condition includes both chronic major depressive disorder and the previous dysthymic disorder. The reason for this change is that there was no evidence for meaningful differences between these two conditions.

The term is from Ancient Greek , meaning bad state of mind.

"Mood disorder due to a general medical condition" is used to describe manic or depressive episodes which occur secondary to a medical condition. There are many medical conditions that can trigger mood episodes, including neurological disorders (e.g. dementias), metabolic disorders (e.g. electrolyte disturbances), gastrointestinal diseases (e.g. cirrhosis), endocrine disease (e.g. thyroid abnormalities), cardiovascular disease (e.g. heart attack), pulmonary disease (e.g. chronic obstructive pulmonary disease), cancer, and autoimmune diseases (e.g. rheumatoid arthritis).

Postpartum depression onset usually begins between two weeks to a month after delivery. Recent studies have shown that 50% of postpartum depressive episodes begin prior to delivery. Therefore, in the DSM-5 postpartum depression is diagnosed under "depressive disorder with peripartum onset", in which "peripartum onset" is defined as anytime either during pregnancy or within the four weeks following delivery. PPD may last several months or even a year. Postpartum depression can also occur in women who have suffered a miscarriage.

Treatment-resistance is relatively common in cases of MDD. Rates of total remission following antidepressant treatment are only 50.4%. In cases of depression treated by a primary-care physician, 32% of patients partially responded to treatment and 45% did not respond at all.

Minor depressive disorder, also known as minor depression, is a mood disorder that does not meet the full criteria for major depressive disorder but at least two depressive symptoms are present for two weeks. These symptoms can be seen in many different psychiatric and mental disorders, which can lead to more specific diagnoses of an individual's condition. However, some of the situations might not fall under specific categories listed in the "Diagnostic and Statistical Manual of Mental Disorders". Minor depressive disorder is an example of one of these nonspecific diagnoses, as it is a disorder classified in the DSM-IV-TR under the category Depressive Disorder Not Otherwise Specified (DD-NOS). The classification of NOS depressive disorders is up for debate. Minor depressive disorder as a term was never an officially accepted term, but was listed in Appendix B of the DSM-IV-TR. This is the only version of the DSM that contains the term, as the prior versions and the most recent edition, DSM-5, does not mention it.

A person is considered to have minor depressive disorder if they experience 2 to 4 depressive symptoms, with one of them being either depressed mood or loss of interest or pleasure, during a 2-week period. The person must not have experienced the symptoms for 2 years and there must not have been one specific event that caused the symptoms to arise. Although not all cases of minor depressive disorder are deemed in need of treatment, some cases are treated similarly to major depressive disorder. This treatment includes cognitive behavioral therapy (CBT), anti-depressant medication, and combination therapy. A lot of research supports the notion that minor depressive disorder is an early stage of major depressive disorder, or that it is simply highly predictive of subsequent major depressive disorder.

Depressive mixed states occur when patients experience depression and non-euphoric, usually subsyndromal, hypomania at the same time. As mentioned previously, it is particularly difficult to diagnose BP-II when a patient is in this state.

In a mixed state, mood is depressed, but the following symptoms of hypomania present as well:

- Irritability

- Mental overactivity

- Behavioral overactivity

Mixed states are associated with greater levels of suicidality than non-mixed depression. Antidepressants may increase this risk.

The DSM-5, released in May 2013, separates the mood disorder chapter from the DSM-TR-IV into two sections: Depressive and Related Disorders and Bipolar and Related Disorders. Bipolar Disorders falls in between Depressive Disorders and Schizophrenia Spectrum and Related Disorders “in recognition of their place as a bridge between the two diagnostic classes in terms of symptomatology, family history and genetics” (Ref. 1, p 123). Bipolar Disorders underwent a few changes in the DSM-5, most notably the addition of more specific symptomology related to hypomanic and mixed manic states. Depressive Disorders underwent the most changes, the addition of three new disorders: disruptive mood dysregulation disorder, persistent depressive disorder (previously dysthymia), and premenstrual dysphoric disorder (previously in Appendix B, the section for disorders needing further research). Disruptive mood dysregulation disorder is meant as a diagnosis for children and adolescents who would normally be diagnosed with bipolar disorder as a way to limit the bipolar diagnosis in this age cohort. Major depressive disorder (MDD) also underwent a notable change, in that the bereavement clause has been removed. Those previously exempt from a diagnosis of MDD due to bereavement are now candidates for the MDD diagnosis.

It is during depressive episodes that BP-II patients often seek help. Symptoms may be syndromal or subsyndromal. Depressive BP-II symptoms may include five or more of the below symptoms (at least one of them must be either depressed mood or loss of interest/pleasure). In order to be diagnosed, they need to be present only during the same two-week period, as a change from previous hypomanic functioning:

- Depressed mood most of the day, nearly every day, as indicated by either subjective report (e.g., feels sad, empty, or hopeless) or observation made by others (e.g., appears tearful). (Note: In children and adolescents, can be irritable mood.)

- Markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day (as indicated by either subjective account or observation).

- Significant weight loss when not dieting or weight gain (e.g., a change of more than 5% of body weight in a month), or decrease or increase in appetite nearly every day. (Note: In children, consider failure to make expected weight gain.)

- Insomnia or hypersomnia nearly every day.

- Psychomotor agitation or retardation nearly every day (observable by others; not merely subjective feelings of restlessness or being slowed down).

- Fatigue or loss of energy nearly every day.

- Feelings of worthlessness or excessive or inappropriate guilt nearly every day (not merely self-reproach or guilt about being sick).

- Diminished ability to think or concentrate, possible irritability or indecisiveness, nearly every day (either by subjective account or as observed by others).

- Recurrent thoughts of death (not just fear of dying), recurrent suicidal ideation without a specific plan, a suicide attempt, or a specific plan for committing suicide.

Evidence also suggests that BP-II is strongly associated with atypical depression. Essentially, this means that many BP-II patients exhibit reverse vegetative symptoms. BP-II patients may have a tendency to oversleep and overeat, while typically depressed patients sleep and eat less than usual.

Postpartum blues, commonly known as "baby blues," is a transient postpartum mood disorder characterized by milder depressive symptoms than postpartum depression. This type of depression can occur in up to 80% of all mothers following delivery. Symptoms typically resolve within two weeks. Symptoms lasting longer than two weeks are a sign of a more serious type of depression. Women who experience "baby blues" may have a higher risk of experiencing a more serious episode of depression later on.

The DSM-IV-TR defines Atypical Depression as a subtype of Major Depressive Disorder with Atypical Features, characterized by:

1. Mood reactivity (i.e., mood brightens in response to actual or potential positive events)

2. At least two of the following:

- Significant weight gain or increase in appetite;

- Hypersomnia (sleeping too much, as opposed to the insomnia present in melancholic depression);

- Leaden paralysis (i.e., heavy, leaden feelings in arms or legs);

- Long-standing pattern of interpersonal rejection sensitivity (not limited to episodes of mood disturbance) that results in significant social or occupational impairment.

3. Criteria are not met for With Melancholic Features or With Catatonic Features during the same episode.

Depression is a state of low mood and aversion to activity. It may be a normal reaction to occurring life events or circumstances, a symptom of a medical condition, a side effect of drugs or medical treatments, or a symptom of certain psychiatric syndromes, such as the mood disorders major depressive disorder and dysthymia. Depression in childhood and adolescence is similar to adult major depressive disorder, although young sufferers may exhibit increased irritability or aggressive and self-destructive behavior, rather than the all-encompassing sadness associated with adult forms of depression. Children who are under stress, experience loss, or have attention, learning, behavioral, or anxiety disorders are at a higher risk for depression. Childhood depression is often co-morbid with mental disorders outside of other mood disorders; most commonly anxiety disorder and conduct disorder. Depression also tends to run in families. Psychologists have developed different treatments to assist children and adolescents suffering from depression, though the legitimacy of the diagnosis of childhood depression as a psychiatric disorder, as well as the efficacy of various methods of assessment and treatment, remains controversial.

Atypical depression, or depression with atypical features as it has been known in the DSM IV, is depression that shares many of the typical symptoms of the psychiatric syndromes major depression or dysthymia but is characterized by improved mood in response to positive events. In contrast, people with melancholic depression generally do not experience an improved mood in response to normally pleasurable events. Atypical depression also features significant weight gain or an increased appetite, hypersomnia, a heavy sensation in the limbs and interpersonal rejection sensitivity that results in significant social or occupational impairment.

Despite its name, "atypical" depression does not mean it is uncommon or unusual. The reason for its name is twofold: (1) it was identified with its "unique" symptoms subsequent to the identification of melancholic depression and (2) its responses to the two different classes of antidepressants that were available at the time were different from melancholic depression (i.e., MAOIs had clinically significant benefits for atypical depression, while tricyclics did not).

Atypical depression is two to three times more common in women than in men. Individuals with atypical features tend to report an earlier age of onset (e.g. while in high school) of their depressive episodes, which also tend to be more chronic and only have partial remission between episodes. Younger individuals may be more likely to have atypical features, whereas older individuals may more often have episodes with melancholic features. Atypical depression has high comorbidity of anxiety disorders, carries more risk of suicidal behavior, and has distinct personality psychopathology and biological traits.

Atypical depression is more common in individuals with bipolar I, bipolar II, cyclothymia and seasonal affective disorder. Depressive episodes in bipolar disorder tend to have atypical features, as does depression with seasonal patterns.

According to the DSM-IV, children must exhibit either a depressed mood or a loss of interest or pleasure in normal activities. These activities may include school, extracurricular activities, or peer interactions. Depressive moods in children can be expressed as being unusually irritable, which may be displayed by "acting out," behaving recklessly, or often reacting with anger or hostility. Children who do not have the cognitive or language development to properly express mood states can also exhibit their mood through physical complaints such as showing sad facial expressions (frowning) and poor eye contact. A child must also exhibit four other symptoms in order to be clinically diagnosed. However, according to the Omnigraphics "Health References Series: Depression Sourcebook, Third Edition", a more calculated evaluation must be given by a medical or mental health professional such as a physiologist or psychiatrist. Following the bases of symptoms, signs include, but are not limited to, an unusual change in sleep habits (for example, trouble sleeping or overly indulged sleeping hours); a significant amount of weight gain/loss by lack or excessive eating; experiencing aches/pains for no apparent reason that can found; and an inability to concentrate on tasks or activities. If these symptoms are present for a period of two weeks or longer, it is safe to make the assumption that the child, or anybody else for that matter, is falling into major depression.

Symptoms of the depressive phase of bipolar disorder include persistent feelings of sadness, irritability or anger, loss of interest in previously enjoyed activities, excessive or inappropriate guilt, hopelessness, sleeping too much or not enough, changes in appetite and/or weight, fatigue, problems concentrating, self-loathing or feelings of worthlessness, and thoughts of death or suicide. In severe cases, the individual may develop symptoms of psychosis, a condition also known as severe bipolar disorder with psychotic features. These symptoms include delusions and hallucinations. A major depressive episode persists for at least two weeks, and may result in suicide if left untreated.

The earlier the age of onset, the more likely the first few episodes are to be depressive. Since a diagnosis of bipolar disorder requires a manic or hypomanic episode, many affected individuals are initially misdiagnosed as having major depression and then incorrectly treated with prescribed antidepressants.

Mania is a distinct period of at least one week of elevated or irritable mood, which can range from euphoria to delirium, and those experiencing hypo- or mania may exhibit three or more of the following behaviors: speak in a rapid, uninterruptible manner, short attention span, racing thoughts, increased goal-oriented activities, agitation, or they may exhibit behaviors characterized as impulsive or high-risk, such as hypersexuality or excessive spending. To meet the definition for a manic episode, these behaviors must impair the individual's ability to socialize or work. If untreated, a manic episode usually lasts three to six months.

People with hypomania or mania may experience a decreased need of sleep, impaired judgment, and speak excessively and very rapidly. Manic individuals often have a history of substance abuse developed over years as a form of "self-medication". At the more extreme, a person in a full blown manic state can experience psychosis; a break with reality, a state in which thinking is affected along with mood. They may feel unstoppable, or as if they have been "chosen" and are on a "special mission", or have other grandiose or delusional ideas. This may lead to violent behavior and, sometimes, hospitalization in an inpatient psychiatric hospital. The severity of manic symptoms can be measured by rating scales such as the Young Mania Rating Scale, though questions remain about the reliability of these scales.

The onset of a manic or depressive episode is often foreshadowed by sleep disturbances. Mood changes, psychomotor and appetite changes, and an increase in anxiety can also occur up to three weeks before a manic episode develops.