impromptu
Concise Oxford English Dictionary © 2008 Oxford University Press:
impromptu /ɪmˈprɒm(p)tjuː/
▶adjective & adverb unplanned or unrehearsed. ▶noun (pl. impromptus) a short piece of instrumental music, especially a solo, reminiscent of an improvisation.
'impromptu' also found in these Oxford entries:

