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):

improvised
    adj 1: done or made using whatever is available; "crossed the
           river on improvised bridges"; "the survivors used jury-
           rigged fishing gear"; "the rock served as a makeshift
           hammer" [syn: improvised, jury-rigged, makeshift]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "improvised":
   ad hoc, ad libitum, ad-lib, band-aid, caught napping,
   caught off balance, caught short, extemporaneous, extemporary,
   extempore, extemporized, haphazard, hasty, impromptu,
   improvisational, improvisatorial, improvise, improviso,
   jury-rigged, makeshift, makeshifty, off-the-cuff, offhand,
   precipitate, provisional, rough-and-ready, snap, stopgap,
   surprised, taken aback, taken by surprise, taken unawares,
   temporary, tentative, tripped up, unarranged, unbegun, unconcocted,
   uncontrived, undeliberated, undevised, unhatched, unmade,
   unmanufactured, unorganized, unplanned, unpremeditated, unprepared,
   unprimed, unready, unrehearsed, unstudied