The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Chem.) An element, atom, or radical having a valence or
      combining power of two.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dyad
    n 1: two items of the same kind [syn: couple, pair,
         twosome, twain, brace, span, yoke, couplet,
         distich, duo, duet, dyad, duad]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "dyad":
   both, brace, couple, couplet, distich, double harness, doublet,
   duad, duet, duo, match, mates, pair, set of two, span, team,
   the two, twain, two, twosome, yoke