Years ago, terms such as "feebleminded, mental defective, idiot and moron" had functional meanings and were acceptable medical terms. Initially, "retardation" replaced older terms because it was less stigmatizing. However, by the 1960s, that term was also fostering stigma. Today, an attitude shift is taking place toward people with intellectual disabilities, and new terms reflect positive changes and avoid stigma. Reject depersonalizing phrases such as "the mentally retarded" in favor of person-first language such as "person who has an intellectual disability".
For more details, see the archived post, “A Brief History of the R-Word”