The Collaborative International Dictionary of English v.0.48:

Triad \Tri"ad\, n. [L. trias, -adis, Gr. ?, ?, fr.?, ?, three:
   cf. F. triade. See Three, and cf. Trias, Trio.]
   1. A union of three; three objects treated as one; a ternary;
      a trinity; as, a triad of deities.
      [1913 Webster]

   2. (Mus.)
      (a) A chord of three notes.
      (b) The common chord, consisting of a tone with its third
          and fifth, with or without the octave.
          [1913 Webster]

   3. (Chem.) An element or radical whose valence is three.
      [1913 Webster]

   Triads of the Welsh bards, poetical histories, in which the
      facts recorded are grouped by threes, three things or
      circumstances of a kind being mentioned together.

   Hindu triad. See Trimurti.
      [1913 Webster]

WordNet (r) 3.0 (2006):

triad
    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: a set of three similar things considered as a unit [syn:
       trio, triad, triplet, triple]
    3: three people considered as a unit [syn: trio, threesome,
       triad, trinity]
    4: a three-note major or minor chord; a note and its third and
       fifth tones [syn: common chord, triad]