A Dyad

1. A set of two elements treated as one; a pair. Synonyms: couple, duad
2. (sociology) Two persons in an ongoing relationship; dyadic relationship.
3. (sociology) The relationship or interaction itself in reference to a couple.
4. (music) Any set of two different pitch classes.
5. (chemistry) An element, atom, or radical having a valence of or combining power of two.
6. (biology) A chromosome structure, usually X- or V-shaped, consisting of two condensed sister chromatids joined by a centromere.
7. (biology) A secondary unit of organisation consisting of an aggregate of monads.
8. (mathematics) A tensor of order two and rank one.