idiocy vs mental retardation

idiocy and mental retardation both are nouns.

Word	Noun	Adjective	Verb	Adverb
idiocy	Yes	No	No	No
mental retardation	Yes	No	No	No

As nouns, mental retardation is a hypernym of idiocy; that is, mental retardation is a word with a broader meaning than idiocy:

Other hypernyms of idiocy include backwardness, retardation, slowness, subnormality.

idiocy (noun)	mental retardation (noun)
extreme mental retardation	lack of normal development of intellectual capacities