Source : Webster's Revised Unabridged Dictionary (1913)

Improvisation \Im*prov`i*sa"tion\, n. [Cf. F. improvisation.]
   1. The act or art of composing and rendering music, poetry,
      and the like, extemporaneously; as, improvisation on the
      organ.

   2. That which is improvised; an impromptu.

Source : WordNet®

improvisation
     n 1: a creation spoken or written or composed extemporaneously
          (without prior preparation)
     2: an unplanned expedient [syn: {temporary expedient}]
     3: a performance given extempore without planning or
        preparation [syn: {extemporization}, {extemporisation}]