Source : Webster's Revised Unabridged Dictionary (1913)

Improvise \Im`pro*vise"\, v. t. [imp. & p. p. {Improvised}; p.
   pr. & vb. n. {Improvising}.] [F. improviser, it.
   improvvisare, fr. improvviso unprovided, sudden, extempore,
   L. improvisus; pref. im- not + provisus foreseen, provided.
   See {Proviso}.]
   1. To compose, recite, or sing extemporaneously, especially
      in verse; to extemporize; also, to play upon an
      instrument, or to act, extemporaneously.

   2. To bring about, arrange, or make, on a sudden, or without
      previous preparation.

            Charles attempted to improvise a peace. --Motley.

   3. To invent, or provide, offhand, or on the spur of the
      moment; as, he improvised a hammer out of a stone.

Improvise \Im`pro*vise"\, v. i.
   To produce or render extemporaneous compositions, especially
   in verse or in music, without previous preparation; hence, to
   do anything offhand.

Source : WordNet®

improvise
     v 1: perform without preparation; "he extemporized a speech at
          the wedding" [syn: {improvize}, {ad-lib}, {extemporize},
           {extemporise}]
     2: manage in a makeshift way; do with whatever is at hand;
        "after the hurricane destroyed our house, we had to
        improvise for weeks" [syn: {extemporize}]