adverb without advance preparation.
— synonyms: ad_lib, ad_libitum, spontaneously
adjective with little or no preparation or forethought.
— synonyms: ad-lib, extemporaneous, extemporary, extempore, offhand, offhanded, off-the-cuff, unrehearsed
noun an extemporaneous speech or remark.
noun a short musical passage that seems to have been made spontaneously without advance preparation.
Examples
“The party began with an impromptu rendition of 'Happy Birthday'.”