Dyad
Dyad , noun
[Latin dyas, dyadis, the number two. Greek {not transcribed}: compare French dyade. See two, and compare Duad.]
1.
Two units treated as one; a couple; a pair.
2.
(Chemistry) An element, atom, or radical having a valence or combining power of two.
Dyad , adjective
(Chemistry) 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.