What is the meaning of impromptu?

noun

• An extemporaneous speech or remark.
  • usage: "a witty impromptu must not sound premeditated"
• A short musical passage that seems to have been made spontaneously without advance preparation.

adjective

• With little or no preparation or forethought.
  • usage: "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"
  • synonyms: ad-lib, extemporaneous, extemporary, extempore, offhand, offhanded, off-the-cuff, unrehearsed

adverb

• Without advance preparation.
  • usage: "he spoke ad lib"
  • synonyms: ad lib, ad libitum, spontaneously