extemporaneous vs ad-lib

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

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

As adjectives, extemporaneous and ad-lib are synonyms defined as:

Other synonyms of extemporaneous include extemporary, extempore, impromptu, off-the-cuff, offhand, offhanded, unrehearsed.

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

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

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