extemporaneous vs impromptu

extemporaneous is not a noun while impromptu is a noun.

Word	Noun	Adjective	Verb	Adverb
extemporaneous	No	Yes	No	No
impromptu	Yes	Yes	No	Yes

As adjectives, extemporaneous and impromptu are synonyms defined as:

extemporaneous and impromptu: with little or no preparation or forethought

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

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

extemporaneous (adjective)	impromptu (adjective)
with little or no preparation or forethought	with little or no preparation or forethought

extemporaneous (adverb)	impromptu (adverb)
	without advance preparation