Dyad

noun

1. A set of two elements treated as one; a pair.
2. Any set of two different pitch classes.
3. A pair of things standing in particular relation; dyadic relation.

   ""For each individual in a specific dyad (i.e., mother-offspring, offspring-father, sibling-sibling),..." Debra Lieberman, John Tooby, and Leda Cosmides - The evolution of human incest avoidance mechanisms: an evolutionary psychological approach, p. 20"

4. An element, atom, or radical having a valence or combining power of two.
5. A secondary unit of organisation consisting of an aggregate of monads.