The Collaborative International Dictionary of English v.0.48:

Triplet \Trip"let\, n. [From Triple.]
   1. A collection or combination of three of a kind; three
      united.
      [1913 Webster]

   2. (Poetry) Three verses rhyming together.
      [1913 Webster]

   3. (Mus.) A group of three notes sung or played in the tree
      of two.
      [1913 Webster]

   4. pl. Three children or offspring born at one birth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

triplet
    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]
    2: one of three offspring born at the same time from the same
       pregnancy
    3: a set of three similar things considered as a unit [syn:
       trio, triad, triplet, triple]