Definitions
1.nounA set of two elements treated as one; a pair.
2.nounAny set of two different pitch classes.
3.nounA 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.nounAn element, atom, or radical having a valence or combining power of two.
5.nounA secondary unit of organisation consisting of an aggregate of monads.
6.nounA tensor of order two and rank one.