decelerate vs decrease

decelerate is not a noun while decrease is a noun.

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

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

Other hypernyms of decelerate include diminish, fall, lessen.

decelerate (noun)	decrease (noun)
	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

decelerate (verb)	decrease (verb)
lose velocity; move more slowly	decrease in size, extent, or range
reduce the speed of	make become smaller