The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. To bring about, arrange, do, or make, immediately or on
      short notice, without previous preparation and with no
      known precedent as a guide.
      [1913 Webster +PJC]

            Charles attempted to improvise a peace. --Motley.
      [1913 Webster]

   3. To invent, or provide, offhand, or on the spur of the
      moment; as, he improvised a hammer out of a stone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

improvise
    v 1: perform without preparation; "he extemporized a speech at
         the wedding" [syn: improvise, 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: improvise, extemporize]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "improvise":
   ad lib, ad-lib, be caught napping, be surprised, be taken unawares,
   be unprepared, beget, breed, bring forth, bring into being,
   call into being, coin, conceive, concoct, contrive, cook up,
   dash off, design, develop, devise, discover, do offhand, dream up,
   engender, evolve, extemporize, fabricate, fake, frame, generate,
   give being to, give rise to, go off half-cocked, hatch,
   have no plan, improvisate, invent, jury-rig, knock off, lash up,
   make do, make do with, make up, mature, mint, originate, plan,
   play by ear, procreate, scrap the plan, spawn, strike off,
   strike out, think out, think up, throw off, throw together,
   toss off, toss out, vamp, whip up, whomp up, wing it