dyad

Etymology

From Ancient Greek δυάς (duás), δυάδ- (duád-) from δύο (dúo, “two”), from Proto-Indo-European *duwó, *duwéh₃ (*dwóh₁). The mathematics sense was coined by American scientist Josiah Willard Gibbs in 1884 in the second half of his book Elements of Vector Analysis.

noun

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, W. D. Ross, Metaphysics Book I, translation of original by Aristotle
   McNamee describes their grip on the company as “the most centralized decision-making structure I have ever encountered in a large company.” Their power dyad is possible only because Facebook’s “core platform,” as McNamee puts it, is relatively simple: It “consists of a product and a monetization scheme.” 2019-01-29, Tom Bissell, “An Anti-Facebook Manifesto”, in New York Times
2. (sociology) Two persons in an ongoing relationship; dyadic relationship.
   For each individual in a specific dyad (i.e., mother-offspring, offspring-father, sibling-sibling), […] 2003, Debra Lieberman, John Tooby, Leda Cosmides, The evolution of human incest avoidance mechanisms[…], page 20
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.