Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   3. (Chem.) An element or radical whose valence is three.

   {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}.

Source : WordNet®

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}, {trine}, {trinity}, {ternary}, {ternion},
           {triplet}, {tercet}, {terzetto}, {trey}, {deuce-ace}]
     2: a set of three similar things considered as a unit [syn: {trio},
         {triplet}]
     3: three people considered as a unit [syn: {trio}, {threesome},
         {trinity}]
     4: a 3-note major or minor chord; a note and its third and
        fifth tones [syn: {common chord}]