ad-lib vs unwritten

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

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

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

ad-lib and unwritten: said or done without having been planned or written in advance

Other synonyms of ad-lib include spontaneous.

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

ad-lib (adjective)	unwritten (adjective)
with little or no preparation or forethought	based on custom rather than documentation
said or done without having been planned or written in advance	using speech rather than writing
	said or done without having been planned or written in advance

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