The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. That which is improvised; an impromptu.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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