An idiot, dolt, dullard or (archaically) mome is a person perceived to be lacking intelligence, or someone who acts in a self-defeating or significantly counterproductive way. Along with the similar terms moron, imbecile, and cretin, the word archaically referred to the intellectually disabled, but have all since gained specialized meanings in modern times. An idiot is said to be idiotic, and to suffer from idiocy.

Moron is a term once used in psychology and psychiatry to denote mild intellectual disability. The term was closely tied with the American eugenics movement. Once the term became popularized, it fell out of use by the psychological community, as it was used more commonly as an insult than as a psychological term. It is similar to imbecile and idiot.

The term imbecile was once used by psychiatrists to denote a category of people with moderate to moderate intellectual disability, as well as a type of criminal. The word arises from the Latin word "imbecillus", meaning weak, or weak-minded. It included people with an IQ of 26–50, between "idiot" (IQ of 0–25) and "moron" (IQ of 51–70). In the obsolete medical classification (ICD-9, 1977), these people were said to have "moderate mental retardation" or "moderate mental subnormality" with IQ of 35–49.

The meaning was further refined into mental and moral imbecility. The concepts of "moral insanity", "moral idiocy"," and "moral imbecility", led to the emerging field of eugenic criminology, which held that crime can be reduced by preventing "feeble-minded" people from reproducing.

"Imbecile" as a concrete classification was popularized by psychologist Henry H. Goddard and was used in 1927 by United States Supreme Court Justice Oliver Wendell Holmes Jr. in his ruling in the forced-sterilization case "Buck v. Bell", 274 U.S. 200 (1927).

The concept is closely associated with psychology, psychiatry, criminology, and eugenics. However, the term "imbecile" quickly passed into vernacular usage as a derogatory term, and fell out of professional use in the 20th century in favor of "mental retardation".

In recent decades, the phrases "mental retardation", "mentally retarded", and "retarded" initially used in a medical manner, are regarded as derogatory and politically incorrect much like "moron", "imbecile", "cretin", "dolt" and "idiot", formerly used as scientific terms in the early 20th century. On October 5, 2010, President of the United States Barack Obama signed Senate Bill 2781, known as "Rosa's Law", which changed references in many Federal statutes that referred to "mental retardation" to refer instead to "intellectual disability".

The term feeble-minded was used from the late nineteenth century in Europe, the United States and Australasia for disorders later referred to as illnesses or deficiencies of the mind.

At the time, "mental deficiency" encompassed all degrees of educational and social deficiency. Within the concept of mental deficiency, researchers established a hierarchy, ranging from idiocy, at the most severe end of the scale; to imbecility, at the median point; and to feeble-mindedness at the highest end of functioning. The latter was conceived of as a form of high-grade mental deficiency.

The development of the ranking system of mental deficiency has been attributed to Sir Charles Trevelyan in 1876, and was associated with the rise of eugenics. The term and hierarchy had been used in that sense at least ten years previously. "Wild card" terms outside the established hierarchy such as "idiot savant", may have been used as connotations for varying degrees of autism.

"Moron" was coined in 1910 by psychologist Henry H. Goddard from the Ancient Greek word "" ("moros"), which meant "dull" and used to describe a person with a mental age in adulthood of between 8 and 12 on the Binet scale. It was once applied to people with an IQ of 51–70, being superior in one degree to "imbecile" (IQ of 26–50) and superior in two degrees to "idiot" (IQ of 0–25). The word "moron", along with others including, "idiotic", "imbecilic", "stupid", and "feeble-minded", was formerly considered a valid descriptor in the psychological community, but it is now deprecated in use by psychologists.

In the obsolete medical classification (ICD-9, 1977), these people (morons and feeble-minded) were said to have "mild mental retardation", "mild mental subnormality" or "high-grade defect" with IQ 50–70.

Following opposition to Goddard's attempts to popularize his ideas, Goddard recanted his earlier assertions about the moron: "It may still be objected that moron parents are likely to have imbecile or idiot children. There is not much evidence that this is the case. The danger is probably negligible."

Intellectual disability (ID) begins during childhood and involves deficits in mental abilities, social skills, and core activities of daily living (ADLs) when compared to same-aged peers. There often are no physical signs of mild forms of ID, although there may be characteristic physical traits when it is associated with a genetic disorder (e.g., Down syndrome).

The level of impairment ranges in severity for each person. Some of the early signs can include:

- Delays in reaching or failure to achieve milestones in motor skills development (sitting, crawling, walking)

- Slowness learning to talk or continued difficulties with speech and language skills after starting to talk

- Difficulty with self-help and self-care skills (e.g., getting dressed, washing, and feeding themselves)

- Poor planning or problem solving abilities

- Behavioral and social problems

- Failure to grow intellectually or continued infant-like behavior

- Problems keeping up in school

- Failure to adapt or adjust to new situations

- Difficulty understanding and following social rules

In early childhood, mild ID (IQ 50–69) may not be obvious or identified until children begin school. Even when poor academic performance is recognized, it may take expert assessment to distinguish mild intellectual disability from specific learning disability or emotional/behavioral disorders. People with mild ID are capable of learning reading and mathematics skills to approximately the level of a typical child aged nine to twelve. They can learn self-care and practical skills, such as cooking or using the local mass transit system. As individuals with intellectual disability reach adulthood, many learn to live independently and maintain gainful employment.

Moderate ID (IQ 35–49) is nearly always apparent within the first years of life. Speech delays are particularly common signs of moderate ID. People with moderate intellectual disability need considerable supports in school, at home, and in the community in order to fully participate. While their academic potential is limited, they can learn simple health and safety skills and to participate in simple activities. As adults, they may live with their parents, in a supportive group home, or even semi-independently with significant supportive services to help them, for example, manage their finances. As adults, they may work in a sheltered workshop.

People with severe or profound ID need more intensive support and supervision their entire lives. They may learn some ADLs, but an intellectual disability is considered severe or profound when individuals are unable to independently care for themselves without ongoing significant assistance from a caregiver throughout adulthood. Individuals with profound ID are completely dependent on others for all ADLs and to maintain their physical health and safety, although they may be able to learn to participate in some of these activities to limited degree.

Intellectual disability (ID), also known as general learning disability, and mental retardation (MR), is a generalized neurodevelopmental disorder characterized by significantly impaired intellectual and adaptive functioning. It is defined by an IQ score under 70 in addition to deficits in two or more adaptive behaviors that affect everyday, general living.

Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environments. As a result of this focus on the person's abilities in practice, a person with an unusually low IQ may not be considered to have intellectually disability.

Intellectual disability is subdivided into syndromic intellectual disability, in which intellectual deficits associated with other medical and behavioral signs and symptoms are present, and non-syndromic intellectual disability, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.

Intellectual disability affects about 2–3% of the general population. Seventy-five to ninety percent of the affected people have mild intellectual disability. Non-syndromic or idiopathic cases account for 30–50% of cases. About a quarter of cases are caused by a genetic disorder, and about 5% of cases are inherited from a person's parents. Cases of unknown cause affect about 95 million people as of 2013.

Idiot is a word derived from the Greek , "idiōtēs" ("person lacking professional skill", "a private citizen", "individual"), from , "idios" ("private", "one's own"). In ancient Greece, people who were not capable of engaging in the public sphere were considered "idiotes", in contrast to the public citizen, or "polites". In Latin the word "idiota" ("ordinary person, layman") preceded the Late Latin meaning "uneducated or ignorant person". Its modern meaning and form dates back to Middle English around the year 1300, from the Old French "idiote" ("uneducated or ignorant person"). The related word "idiocy" dates to 1487 and may have been analogously modeled on the words prophet and prophecy. The word has cognates in many other languages.

An idiot in Athenian democracy was someone who was characterized by "self-centeredness" and concerned almost exclusively with "private"—as opposed to "public"—affairs. Idiocy was the natural state of ignorance into which all persons were born and its opposite, citizenship, was effected through formalized education. In Athenian democracy, idiots were "born" and citizens were "made" through education (although citizenship was also largely hereditary). "Idiot" originally referred to a "layman, person lacking professional skill". Declining to take part in public life, such as democratic government of the polis (city state), was considered dishonorable. "Idiots" were seen as having bad judgment in public and political matters. Over time, the term "idiot" shifted away from its original connotation of selfishness and came to refer to individuals with overall bad judgment–individuals who are "stupid". According to the Bauer-Danker Lexicon, the noun ίδιωτής in ancient Greek meant "civilian" (ref Josephus Bell 2 178), "private citizen" (ref sb 3924 9 25), "private soldier as opposed to officer," (Polybius 1.69), "relatively unskilled, not clever," (Herodotus 2,81 and 7 199). The military connotation in Bauer's definition stems from the fact that ancient Greek armies in the time of total war mobilized all male citizens (to the age of 50) to fight, and many of these citizens tended to fight poorly and ignorantly.

The earliest recorded use of the term in the English language dates from 1534, when it appears in one of the first English translations of the New Testament. A biblical injunction to "Comforte the feble mynded" is included in 1 Thessalonians 5:14.

A London "Times" editorial of November 1834 describes the long-serving former Prime Minister Lord Liverpool as a "feeble-minded pedant of office".