mental handicap

Concise Oxford English Dictionary © 2008 Oxford University Press:
mental handicap

▶noun a condition in which the intellectual capacity of a person is permanently lowered or underdeveloped to an extent which prevents normal function in society.

– derivatives
mentally handicapped adverb.

usage: The terms mental handicap and mentally handicapped have fallen out of favour in recent years and have been largely replaced in official contexts by terms such as learning difficulties. See usage at learning difficulties.