Dyad /(?)/

Dy·ad

Dyad

n.
  1. Two units treated as one; a couple; a pair.
  2. An element, atom, or radical having a valence or combining power of two. (Chem.)

Dyad

a.
  1. 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. (Chem.)