delay vs decelerate

delay is a noun but decelerate is not a noun.

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

As verbs, decelerate is a hypernym of delay; that is, decelerate is a word with a broader meaning than delay:

Other hypernyms of delay include retard, slow, slow down, slow up.

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

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