ad-lib vs unrehearsed

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

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

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

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

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

ad-lib (adjective)	unrehearsed (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

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