dual

1. Of an item that is one of a pair, the other item in the pair.
2. 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.

3. (grammar) dual number The grammatical number of a noun marking two of something (as in singular, dual, plural), sometimes referring to two of anything (a couple of, exactly two of), or a chirality-marked pair (as in left and right, as with gloves or shoes) or in some languages as a discourse marker, "between you and me". A few languages display trial number.
4. 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.

verb

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

1. Exhibiting duality; characterized by having two (usually equivalent) components.
2. Acting as a counterpart.
3. Double.

   a dual-headed computer

4. (grammar) Pertaining to grammatical number (as in singular and plural), referring to two of something, such as a pair of shoes, in the context of the singular, plural and, in some languages, trial grammatical number.

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

5. Being the space of all linear functionals of (some other space).
6. Being the dual of some other category; containing the same objects but with source and target reversed for all morphisms.