imbecility vs backwardness

imbecility and backwardness both are nouns.

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

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

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

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

imbecility (noun)	backwardness (noun)
a stupid mistake	lack of normal development of intellectual capacities
retardation more severe than a moron but not as severe as an idiot