The Collaborative International Dictionary of English v.0.48:

Masterpiece \Mas"ter*piece`\, n.
   Anything done or made with extraordinary skill; a capital
   performance; a chef-d'oeuvre; a supreme achievement.
   [1913 Webster]

         The top and masterpiece of art.          --South.
   [1913 Webster]

         Dissimulation was his masterpiece.       --Claredon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

masterpiece
    n 1: the most outstanding work of a creative artist or craftsman
         [syn: masterpiece, chef-d'oeuvre]
    2: an outstanding achievement

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "masterpiece":
   archetype, art object, artifact, brainchild, bric-a-brac, child,
   classic, coinage, composition, concoction, creation, creature,
   crowning achievement, design, distillation, effect, end product,
   essence, exemplar, extract, feat, fruit, grotesque, handiwork,
   ideal, invention, issue, jewel, kitsch, magnum opus, manufacture,
   master, master stroke, masterstroke, masterwork, mintage, mirror,
   mobile, museum piece, new mintage, nude, offspring, old master,
   opera, opus, opuscule, origination, outcome, outgrowth, paragon,
   pasticcio, pastiche, pattern of perfection, piece,
   piece de resistance, piece of virtu, product, production,
   prototype, quintessence, result, stabile, statue, still life,
   study, tour de force, very model, virtu, work, work of art