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.

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".

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.