impromptu

(noun) an extemporaneous speech or remark. Example: "A witty impromptu must not sound premeditated."

(noun) a short musical passage that seems to have been made spontaneously without advance preparation.

(adj) with little or no preparation or forethought. Synonyms: ad-lib, extemporaneous, extemporary, extempore, offhand, offhanded, off-the-cuff, unrehearsed. Examples: "His ad-lib comments showed poor judgment." "An extemporaneous piano recital." "An extemporary lecture." "An extempore skit." "An impromptu speech." "Offhand excuses." "Trying to sound offhanded and reassuring." "An off-the-cuff toast." "A few unrehearsed comments."

(adv) without advance preparation. Synonyms: ad lib, ad libitum, spontaneously. Example: "He spoke ad lib."