an extemporaneous speech or remark; "a witty impromptu must not sound premeditated"

with little or no preparation or forethought; "his ad-lib comments showed poor judgment"; "an extemporaneous piano recital"; "an extemporary lecture"; "an extempore skit"; "an impromptu speech"; "offhand excuses"; "trying to sound offhanded and reassuring"; "an off-the-cuff toast"; "a few unrehearsed comments"

without advance preparation; "he spoke ad lib"

A performance or trick done without any special preparation.

Poem written on the spur of the moment e.g. Impromptu on Mrs Riddell's Birthday by Robert Burns. See also extempore.

An impromptu (loosely meaning "offhand") is a free-form musical composition with the character of an improvisation, usually for a solo instrument, such as piano.

An instrumental work, originally meant to convey an improvised or spontaneous work.