As a Noun: 1. A disability affecting the brain so that a person has a reduced capacity for such things as learning, receptive and expressive language, and independent living. English = mental disability, intellectual disability. As a Verb or Adjective: 1. Of a person, to have a disability affecting the brain so that he/she has a reduced intellectual capacity for such things as learning, receptive and expressive language, and independent living. English = mentally disabled, intellectually disabled.