1. A term for a mentally deficient person whose mental age is equivalent to that of someone between eight and twelve years of age; subnormal intelligence; in England, called, a dullard.
2. Informally, a "stupid" person; a dolt.
3. Someone who is considered to be notably stupid or lacking in good judgment.
4. In psychology, a person of borderline intelligence in a former classification of mental retardation, having an intelligence quotient of 50 to 69; feeble-minded.

The term belongs to a classification system no longer in use and is now considered offensive.

Though a psychological, clinical, and medical term, it is often used as a pejorative term against someone of low intelligence; or even, as just a general pejorative.

Most of those who are clinically classified as being morons are considered to be educable and do not require institutionalization but need some supervision in working at some simple job by which they can become "self-sustaining" members of society.