impromptu(any composition, musical or otherwise, that is created on the spot without preparation) noun [UK: ɪm.ˈprɒmp.tjuː] [US: ˌɪm.ˈprɑːmp.tuː]
improvisation [improvisations](act or art of composing and rendering music, poetry, and the like, extemporaneously) noun [UK: ˌɪm.prə.vaɪ.ˈzeɪʃ.n̩] [US: ˌɪmpra.və.ˈzeɪʃ.n̩]