subnormality vs mental retardation

subnormality and mental retardation both are nouns.

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

As nouns, subnormality and mental retardation are synonyms defined as:

subnormality and mental retardation: lack of normal development of intellectual capacities

Other synonyms of subnormality include backwardness, retardation, slowness.

subnormality (noun)	mental retardation (noun)
lack of normal development of intellectual capacities	lack of normal development of intellectual capacities
the state of being less than normal (especially with respect to intelligence)