Of an item that is one of a pair, the other item in the pair.

(geometry) Of a regular polyhedron with V vertices and F faces, the regular polyhedron having F vertices and V faces.

(Can we clean up(+) this sense?) (grammar) The dual number.

(mathematics) Of a vector in an inner product space, the linear functional corresponding to taking the inner product with that vector. The set of all duals is a vector space called the dual space.

(transitive) To convert from single to dual; specifically, to convert a single-carriageway road to a dual carriageway.

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.