improvise vs ad-lib

improvise is not a noun while ad-lib is a noun.

Word	Noun	Adjective	Verb	Adverb
improvise	No	No	Yes	No
ad-lib	Yes	Yes	Yes	No

As verbs, improvise and ad-lib are synonyms defined as:

Other synonyms of improvise include extemporise, extemporize, improvize.

improvise (noun)	ad-lib (noun)
	remark made spontaneously without prior preparation

improvise (adjective)	ad-lib (adjective)
	with little or no preparation or forethought
	said or done without having been planned or written in advance

improvise (verb)	ad-lib (verb)
perform without preparation	perform without preparation
manage in a makeshift way; do with whatever is at hand