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1660s (adv.), 1764 (adj.), from French impromptu (1650s), from Latin in promptu "in readiness," from assimilated form of in- "into, in, on, upon" (see in- (2)) + promptu, ablative of promptus "readiness," from past participle of promere "to bring out," from pro- "before, forward, for" + emere "to obtain" (see exempt).
a 19th-century piano composition intended to produce the illusion of spontaneous improvisation. In keeping with this fundamental premise, there is no particular form associated with the impromptu, although ternary and rondo schemes are common. The style of the music is similar to that of other compositions of the period, with such designations as fantasie, caprice, and bagatelle.