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