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]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "triplet":
   Spenserian stanza, accidental, andante tempo, antistrophe, beat,
   book, breve, burden, canto, chorus, clover, compound time, couplet,
   crotchet, demisemiquaver, deuce-ace, distich, dominant,
   dominant note, double whole note, duple time, eighth note,
   enharmonic, enharmonic note, envoi, epode, flat, half note,
   hemidemisemiquaver, heptastich, hexastich, largo, leash, line,
   march tempo, measure, minim, mixed times, monostich, musical note,
   natural, note, octastich, octave, octet, ottava rima, patent note,
   pentastich, presto, quarter note, quatrain, quaver, rag, ragtime,
   refrain, report, responding note, rhyme royal, rubato, semibreve,
   semiquaver, septet, sestet, set of three, sextet, sextuple time,
   shamrock, shaped note, sharp, simple time, sixteenth note,
   sixty-fourth note, spiccato, staccato, stanza, stave, strain,
   strophe, sustained note, syllable, syncopation, syncope, tempo,
   tempo rubato, tercet, ternary, ternion, terza rima, terzetto,
   tetrastich, thirty-second note, three, three-quarter time,
   threesome, tierce, time, time pattern, timing, tone, trefoil, trey,
   triad, trialogue, triangle, tricorn, trident, triennium, trihedron,
   trilogy, trimester, trine, trinity, trinomial, trio, triphthong,
   triple crown, triple threat, triple time, triplopy, tripod,
   triptych, trireme, triseme, triskelion, tristich, trisul,
   triumvirate, triunity, trivet, troika, two-four time, verse,
   waltz time, whole note