n. 25B6; adjective an impromptu lecture : UNREHEARSED, unprepared, unscripted, extempore, extemporized, extemporaneous, improvised, spontaneous, unplanned; informal off-the-cuff.
prepared, rehearsed.
25B6; adverb they played the song impromptu : EXTEMPORE, spontaneously, extemporaneously, without preparation, without rehearsal; informal off the cuff, off the top of one's head.