impromptu vs ad-lib

impromptu and ad-lib both are nouns.

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

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

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

impromptu (noun)	ad-lib (noun)
an extemporaneous speech or remark	remark made spontaneously without prior preparation
a short musical passage that seems to have been made spontaneously without advance preparation

impromptu (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

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

impromptu (adverb)	ad-lib (adverb)
without advance preparation