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".

In 19th- and early 20th-century medicine and psychology, an "idiot" was a person with a very profound intellectual disability. In the early 1900s, Dr. Henry H. Goddard proposed a classification system for intellectual disability based on the Binet-Simon concept of mental age. Individuals with the lowest mental age level (less than three years) were identified as "idiots"; "imbeciles" had a mental age of three to seven years, and "morons" had a mental age of seven to ten years. The term "idiot" was used to refer to people having an IQ below 30 IQ, or intelligence quotient, was originally determined by dividing a person's mental age, as determined by standardized tests, by their actual age. The concept of mental age has fallen into disfavor, though, and IQ is now determined on the basis of statistical distributions.

In the obsolete medical classification (ICD-9, 1977), these people were said to have "profound mental retardation" or "profound mental subnormality" with IQ under 20. This term is not in use in the United Kingdom.