ad-lib vs extemporise

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

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

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

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

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

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

ad-lib (verb)	extemporise (verb)
perform without preparation	perform without preparation