The Collaborative International Dictionary of English v.0.48:

Tercet \Ter"cet\, n. [F., fr. It. terzetto, dim. of terzo,
   third, L. tertius. See Tierce, and cf. Terzetto.]
   1. (Mus.) A triplet. --Hiles.
      [1913 Webster]

   2. (Poetry) A triplet; a group of three lines.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tercet
    n 1: the cardinal number that is the sum of one and one and one
         [syn: three, 3, III, trio, threesome, tierce,
         leash, troika, triad, trine, trinity, ternary,
         ternion, triplet, tercet, terzetto, trey, deuce-
         ace]