mental retardation vs subnormality

mental retardation and subnormality both are nouns.

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

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

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

Other synonyms of mental retardation include backwardness, retardation, slowness.

mental retardation (noun)	subnormality (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)