Impromptu

Part of speech: noun (n.)

1. Something made or done offhand, at the moment, or without previous study; an extemporaneous composition, address, or remark.

2. A piece composed or played at first thought; a composition in the style of an extempore piece.

Part of speech: adverb (adv.)

Offhand; without previous study; extemporaneous; extempore; as, an impromptu verse.