adjective
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.
noun
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.
(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.
(wrestling) A head-to-head match or meet between two teams, such as two high schools or colleges.
verb
(transitive) To convert from single to dual; specifically, to convert a single-carriageway road to a dual carriageway.