As nouns the difference between impromptu and impromptus
is that impromptu
is (music) a short musical composition for an informal occasion often with the character of improvisation and usually to be played solo while impromptus
As an adjective impromptu
is improvised; without prior preparation, planning or rehearsal; extemporaneous; unplanned.
As an adjective extemporaneous is
without preparation or advanced thought; offhand.