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Definition of impromptu : 1. A piece composed or played at first thought; a composition in the style of an extempore piece.
2. Offhand; without previous study; extemporaneous; extempore; as, an impromptu verse.
3. Something made or done offhand, at the moment, or without previous study; an extemporaneous composition, address, or remark.

Synonyms:

offhanded, prepared, extemporary, unwritten, ad libitum, off-the-cuff, snap, offhand, spur-of-the-moment, ad-lib, spontaneously, extempore

improvised, unpremeditated, abrupt, impulsive, instantaneous, hasty, unrehearsed, ill-considered, spontaneous, rash, reflexive, extemporaneous

makeshift, undevised, unprepared, rough-and-ready, haphazard

Usage examples:

• Just then, however, he caught sight of the very handsome young lady who was seated in the office, and his whole demeanour underwent a most remarkable change; out came the hands from his pockets, off went the hat, and, turning, he bowed, really rather nicely, considering how impromptu the whole performance was. - "Mr. Meeson's Will", H. Rider Haggard.
• " I forgot my gloves," he said, appearing to take something from the side- table; then, as an impromptu thought, he remarked, " You have no binding engagement at home perhaps, Caroline?" - "Shirley", Charlotte Brontë.
• Yet there are refreshing passages in it, and the youthful English officer who kisses Septimius's sweetheart before his eyes, and afterward fights an impromptu duel with him, dying as cheerfully as he had lived, is an original and charming character. - "The Life and Genius of Nathaniel Hawthorne", Frank Preston Stearns.