Source : Webster's Revised Unabridged Dictionary (1913)

Impromptu \Im*promp"tu\, adv. or a. [F. impromptu, fr. L. in
   promptu in readiness, at hand; in in + promptus visibility,
   readiness, from promptus visible, ready. See {Prompt}.]
   Offhand; without previous study; extemporaneous; extempore;
   as, an impromptu verse.

Impromptu \Im*promp"tu\, n.
   1. Something made or done offhand, at the moment, or without
      previous study; an extemporaneous composition, address, or
      remark.

   2. (Mus.) A piece composed or played at first thought; a
      composition in the style of an extempore piece.

Source : WordNet®

impromptu
     adj : 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" [syn:
            {ad-lib}, {extemporaneous}, {extemporary}, {extempore},
            {offhand}, {offhanded}, {off-the-cuff}, {unrehearsed}]
     n 1: an extemporaneous speech or remark; "a witty impromptu must
          not sound premeditated"
     2: a short musical passage that seems to have been made
        spontaneously without advance preparation
     adv : without advance preparation; "he spoke ad lib" [syn: {ad lib},
            {ad libitum}, {spontaneously}]