dual

Characterized by having two (usually equivalent) components.

Pertaining to two, pertaining to a pair of.

(grammar) Pertaining to a grammatical number in certain languages that refers to two of something, such as a pair of shoes.

(mathematics, physics) Exhibiting duality.

(linear algebra) Being the space of all linear functionals of (some other space).

(category theory) Being the dual of some other category; containing the same objects but with source and target reversed for all morphisms.