Impromptu, adv. or a.
Etymology: [F. impromptu, fr. L. in promptu in readiness, at hand; in in + promptus visibility, readiness, from promptus visible, ready. See Prompt.]
Offhand; without previous study; extemporaneous; extempore; as, an impromptu verse.
1 other Webster entry under this spelling
Impromptu, n.
1. Something made or done offhand, at the moment, or without previous study; an extemporaneous composition, address, or remark.
2. (Mus.)
A piece composed or played at first thought; a composition in the style of an extempore piece.
Shades of meaning
Fernald has no entry of its own for impromptu; he lists it under extemporaneous.
Where he writes about it:
Extemporaneous, originally signifying of or from the time or occasion, has come to mean done or made with but little (if any) preparation, and is now chiefly applied to addresses of which the thought has been prepared, and only the language and incidental treatment left to the suggestion of the moment, so that an extemporaneous speech is understood to be any one that is not read or recited; impromptu keeps its original sense, denoting something that springs from the instant; the impromptu utterance is generally brief, direct, and vigorous; the extemporaneous speech may chance to be prosy. — from extemporaneous
Unpremeditated is graver and more formal, denoting absolute want of preparation, but is rather too heavy a word to be applied to such apt, ready utterance as is generally designated by impromptu. — from extemporaneous