decelerate vs retard

decelerate is not a noun while retard is a noun.

Word	Noun	Adjective	Verb	Adverb
decelerate	No	No	Yes	No
retard	Yes	No	Yes	No

As verbs, decelerate and retard are synonyms defined as:

Other synonyms of decelerate include slow, slow down, slow up.

As verbs, retard is a hyponym of decelerate; that is, retard is a word with a more specific, narrower meaning than decelerate:

Other hyponyms of decelerate include moderate, fishtail.

decelerate (noun)	retard (noun)
	a person of subnormal intelligence

decelerate (verb)	retard (verb)
lose velocity; move more slowly	lose velocity; move more slowly
reduce the speed of	cause to move more slowly or operate at a slower rate
	slow the growth or development of
	be delayed