How do you spell Dyad

Available Definitions:
1)  n. - Two units treated as one; a couple; a pair.
2)  n. - An element, atom, or radical having a valence or combining power of two.
3)  a. - 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 .