The term idiot, dolt, or dullard is a mentally deficient person, or someone who acts in a self-defeating or significantly counterproductive way. Archaically the word mome has also been used. The similar terms moron, imbecile, and cretin have all gained specialized meanings in modern times. An idiot is said to be idiotic, and to suffer from idiocy. A dunce is an idiot who is specifically incapable of learning. An idiot differs from a fool (who is unwise) and an ignoramus (who is uneducated/an ignorant), neither of which refers to someone with low intelligence.
In 19th and early 20th century medicine and psychology, an "idiot" was a person with a very severe mental retardation. In the early 1900s, Dr. Henry H. Goddard proposed a classification system for mental retardation 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. IQ, or intelligence quotient, is determined by dividing a person's mental age, as determined by standardized tests, by their actual age. The term "idiot" was used to refer to people having an IQ below 30. In current medical classification, these people are now said to have profound mental retardation.