In medicine and psychology an idiot is a person with a very severe mental retardation or a very low IQ level. Idiots were defined as people whose IQ were below 20 (with a standard deviation of 16). In the current classification in US the same people are now said to have a profound mental retardation, and the term is no longer used.

Originally, an idiot was a person that was not interested in participating in politics. 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. See Athenian democracy.