retardation vs backwardness

retardation and backwardness both are nouns.

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

As nouns, retardation and backwardness are synonyms defined as:

retardation and backwardness: lack of normal development of intellectual capacities

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

retardation (noun)	backwardness (noun)
the act of slowing down or falling behind	lack of normal development of intellectual capacities
a decrease in rate of change
lack of normal development of intellectual capacities
any agent that retards or delays or hinders
the extent to which something is delayed or held back