dyad

noun8

1. •
   A set of two elements treated as one; a pair.

   • […] positing a dyad and constructing the infinite out of great and small, instead of treating the infinite as one, is peculiar to him; […] — 1908
2. •
   Two persons in an ongoing relationship; a dyadic relationship.

   • For each individual in a specific dyad (i.e., mother-offspring, offspring-father, sibling-sibling), […] — 2003
3. •
   Any set of two different pitch classes.
4. •
   The relationship or interaction itself in reference to a couple.
5. •
   A chromosome structure, usually X- or V-shaped, consisting of two condensed sister chromatids joined by a centromere.
6. •
   An element, atom, or radical having a valence of or combining power of two.
7. •
   A tensor of order two and rank one.
8. •
   A secondary unit of organisation consisting of an aggregate of monads.