mental retardation vs backwardness

mental retardation and backwardness both are nouns.

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

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

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

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

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