Source : Webster's Revised Unabridged Dictionary (1913)

Dyad \Dy"ad\, n. [L. dyas, dyadis, the number two. Gr. ?: cf. F.
   dyade. See two, and cf. {Duad}.]
   1. Two units treated as one; a couple; a pair.

   2. (Chem.) An element, atom, or radical having a valence or
      combining power of two.

Dyad \Dy"ad\, a. (Chem.)
   Having a valence or combining power of two; capable of being
   substituted for, combined with, or replaced by, two atoms of
   hydrogen; as, oxygen and calcium are dyad elements. See
   {Valence}.

Source : WordNet®

dyad
     n : two items of the same kind [syn: {couple}, {pair}, {twosome},
          {twain}, {brace}, {span}, {yoke}, {couplet}, {distich},
         {duo}, {duet}, {duad}]