ad-lib vs improvise

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

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

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

Other synonyms of ad-lib include extemporise, extemporize, improvize.

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

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

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