backwardness vs mental retardation

backwardness and mental retardation both are nouns.

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

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

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

Other synonyms of backwardness include retardation, slowness, subnormality.

backwardness (noun)	mental retardation (noun)
lack of normal development of intellectual capacities	lack of normal development of intellectual capacities