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.
© Webster 1913.
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.
© Webster 1913.