impromptu

nouns

an extemporaneous speech or remark
"a witty impromptu must not sound premeditated"
impromptu
is a type of (narrower)
the act of delivering a formal spoken communication to an audience
"he listened to an address on minor Roman poets"
address, speech
a short musical passage that seems to have been made spontaneously without advance preparation
impromptu
is a type of (narrower)
a short section of a musical composition
musical passage, passage

adjectives

with little or no preparation or forethought
"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"
ad-lib, extemporaneous, extemporary, extempore, impromptu, off-the-cuff, offhand, offhanded, unrehearsed

adverbs

without advance preparation
"he spoke ad lib"
ad lib, ad libitum, impromptu, spontaneously