dyad

Dyad refers to a pair of related items treated as a unit. The term derives from the Greek dyás meaning two. In general usage a dyad is a two-element set or ordered pair; in different fields it denotes a two-part unit appropriate to the discipline.

In sociology and psychology, a dyad denotes a two-person relationship or interaction, such as a romantic couple, a parent–child pair, or any direct two-person communication. Dyads are often studied to understand interpersonal dynamics, communication patterns, and reciprocity. Dyadic data analysis is a common method.

Mathematics and physics use dyad to describe a second-order tensor formed by the outer product of two vectors. A dyadic product uv^T yields a rank-1 tensor. The term dyadic is used for objects that can be expressed as a sum of such outer products; in three-dimensional space a dyad often encodes directional information, such as stress or rate-of-deformation tensors.

In chemistry and related fields, dyad is used occasionally to denote a two-atom fragment or a two-component unit within a larger molecule, though this usage is less standard than other nomenclature.

Other contexts include linguistics and archaeology where dyads describe paired elements or two-part structures. The term is often contrasted with monad (a single unit) and triad (a group of three).