impromptu

Etymology

Borrowed from French impromptu.

adj

1. Improvised; without prior preparation, planning or rehearsal.
   The party began with an impromptu rendition of 'Happy Birthday'.
   an impromptu speech

noun

1. (music) A short musical composition for an informal occasion often with the character of improvisation and usually to be played solo.
   The second impromptu is a dance-like Allegretto in A flat major, with a trio in D flat major employing arpeggiated textures. 1997, Christopher H. Gibbs, The Cambridge Companion to Schubert, Cambridge University Press, page 168
2. (by extension) Any composition, musical or otherwise, that is created on the spot without preparation.