The Collaborative International Dictionary of English v.0.48:

Virtuoso \Vir`tu*o"so\, n.; pl. Virtuosos; It. Virtuosi.
   [It. See Virtuous.]
   1. One devoted to virtu; one skilled in the fine arts, in
      antiquities, and the like; a collector or ardent admirer
      of curiosities, etc.
      [1913 Webster]

            Virtuoso the Italians call a man who loves the noble
            arts, and is a critic in them.        --Dryden.
      [1913 Webster]

   2. (Mus.) A performer on some instrument, as the violin or
      the piano, who excels in the technical part of his art; a
      brilliant concert player.
      [1913 Webster]

WordNet (r) 3.0 (2006):

virtuoso
    adj 1: having or revealing supreme mastery or skill; "a
           consummate artist"; "consummate skill"; "a masterful
           speaker"; "masterful technique"; "a masterly performance
           of the sonata"; "a virtuoso performance" [syn:
           consummate, masterful, masterly, virtuoso(a)]
    n 1: someone who is dazzlingly skilled in any field [syn: ace,
         adept, champion, sensation, maven, mavin,
         virtuoso, genius, hotshot, star, superstar,
         whiz, whizz, wizard, wiz]
    2: a musician who is a consummate master of technique and
       artistry

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "virtuoso":
   A per se, Daedalian, ace, adept, adroit, amateur, apt, arbiter,
   arbiter elegantiarum, arbiter of taste, artist, artiste, artistic,
   authoritative, authority, bon vivant, boss, bravura, brilliant,
   champion, chief, clean, clever, cognoscente, collector, commander,
   concert artist, connaisseur, connoisseur, coordinated, crack,
   crackerjack, critic, cunning, cute, dabster, daedal, dazzling,
   dean, deft, dexterous, dextrous, dilettante, diplomatic, epicure,
   epicurean, excellent, exceptional, executant, expert,
   extraordinary, fancy, first-rate, first-rater, fugleman, genius,
   good, good hand, good judge, goodish, gourmand, gourmet, graceful,
   great, handy, head, higher-up, ingenious, interpreter, judge,
   laureate, leader, leading light, luminary, maestro, magician,
   magisterial, mahatma, man of genius, master, master hand,
   masterful, masterly, mastermind, matchless, maven, minstrel,
   minstrelsy, music maker, music-loving, musical, musically inclined,
   musician, musicianly, musicophile, neat, no mean, nonpareil,
   old hand, paragon, past master, peerless, performer, philharmonic,
   player, politic, practiced hand, principal, prodigious, prodigy,
   professional, proficient, quick, quite some, ready, refined palate,
   remarkable, resourceful, ruler, sage, senior, skilled hand,
   skillful, slick, soloist, some, star, statesmanlike, sterling,
   stylish, superb, superior, superlative, superman, superstar,
   tactful, talent, talented, the compleat, the complete,
   the greatest, the most, top dog, topnotcher, tunester, virtuosa,
   virtuose, virtuosic, well-done, whiz, wizard, wonder,
   workmanlike