decelerate vs delay

decelerate is not a noun while delay is a noun.

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

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

Other hyponyms of decelerate include slow, slow down, slow up, detain, hold up.

decelerate (noun)	delay (noun)
	the act of delaying; inactivity resulting in something being put off until a later time
	time during which some action is awaited

decelerate (verb)	delay (verb)
lose velocity; move more slowly	cause to be slowed down or delayed
reduce the speed of	stop or halt
	slow the growth or development of
	act later than planned, scheduled, or required