Source : Webster's Revised Unabridged Dictionary (1913)

Duad \Du"ad\, n. [See {Dyad}.]
   A union of two; duality. [R.] --Harris.

Source : WordNet®

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