imbecility vs subnormality

imbecility and subnormality both are nouns.

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

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

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

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

imbecility (noun)	subnormality (noun)
a stupid mistake	lack of normal development of intellectual capacities
retardation more severe than a moron but not as severe as an idiot	the state of being less than normal (especially with respect to intelligence)