imbecility vs retardation

imbecility and retardation both are nouns.

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

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

imbecility: retardation more severe than a moron but not as severe as an idiot
retardation: lack of normal development of intellectual capacities

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

imbecility (noun)	retardation (noun)
a stupid mistake	the act of slowing down or falling behind
retardation more severe than a moron but not as severe as an idiot	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