impromptu

(ĭm-prŏmpˈto͞o, -tyo͞o)

1. Prompted by the occasion rather than being planned in advance: an impromptu party.
2. Spoken, performed, done, or composed with little or no preparation; extemporaneous: a few impromptu remarks. See Synonyms at extemporaneous.

1. Something, such as a speech, that is made or done extemporaneously.
2. Music A short composition, especially for the piano, performed in an offhand or extemporized style.