Impromptu means performed done or made without previous preparation.
Impromptu comes under adjective.
Synonyms: spontaneous, extemporaneous.
Tip: Impromptu and extemporaneous mean expressed or done with little or no preparation. Extemporaneous most often refers to speech delivered without the assistance of written text, though it may have been planned. Impromptu more often suggests action that is even more spur of the moment or in response to an unforeseen need.
My sister entertained the guests with an impromptu classical dance.
Mr Ravi said that her acceptance speech was impromptu, but I saw him memorizing his notes in the bedroom.