[n] an extemporaneous speech or remark; "a witty impromptu must not sound premeditated"
[n] a short musical passage that seems to have been made spontaneously without advance preparation
[adv] without advance preparation; "he spoke ad lib"
[adj] in response to an unforeseen need; "an impromptu speech"; "a spur-of-the-moment party"
Poem written on the spur of the moment e.g. Impromptu on Mrs Riddell's Birthday by Robert Burns. See also extempore.
ad lib, ad libitum, spontaneously, spur-of-the-moment, unprepared
address, musical passage, passage, speech