Dyad

Part of speech: noun (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.

Part of speech: adjective (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.