Idiocy indicates the severest form of mental deficiency, where the mental age is 2 years or less, and the person cannot guard himself against common physical dangers. The term is gradually being replaced by the term severe subnormality.

Idiot is the corresponding term for a person affected by idiocy. Similarly, the terms imbecile and moron once had scientific definitions that have now been superseded.

Imbecility was a type of mental deficiency less severe than idiocy and not inherited.

A moron was defined by the American Association for the Study of the Feeble-minded in 1910, following work by H. H. Goddard, as an adult with a mental age between eight and twelve. Alternative definitions of these terms based on IQ were also used. For example, the following data based on the Wechsler adult IQ test (WAIS) were used in 1958:

class	IQ
idiot	below 29
imbecile	30-49
moron	50-69
borderline deficiency	70-79

The word idiot comes from the Greek word ιδιωτης, idiôtês, "a private citizen, individual", from ιδιος, idios, "private". In ancient Athens, an idiot was a person who declined to take part in public life, such as democratic city government. Since such activities were honorable and could directly affect all citizens, idiot was a term of derision.

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