(1):
(n.) A piece composed or played at first thought; a composition in the style of an extempore piece.
(2):
(adv. / a.) Offhand; without previous study; extemporaneous; extempore; as, an impromptu verse.
(3):
(n.) Something made or done offhand, at the moment, or without previous study; an extemporaneous composition, address, or remark.