idiocy vs retardation

idiocy and retardation both are nouns.

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

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

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

idiocy (noun)	retardation (noun)
extreme mental retardation	the act of slowing down or falling behind
	a decrease in rate of change
	lack of normal development of intellectual capacities
	any agent that retards or delays or hinders
	the extent to which something is delayed or held back