dual.

Characterized by having two (usually equivalent) components.

a dual-motor vehicle

Pertaining to two, pertaining to a pair of.

dual engine failure

dual citizenship

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

Modern Arabic displays a dual number, as did Homeric Greek.

Exhibiting duality.

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

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

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

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

The octahedron is the dual of the cube. The icosahedron is the dual of the dodecahedron.

The dual number.

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.

A head-to-head match or meet between two teams, such as two high schools or colleges.

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