Dual

Latin dualis ("two"), from duo ("two"), + adjective suffix -alis

Full definition of dual

Exhibiting duality; characterized by having two (usually equivalent) components.
Acting as a counterpart.
Double.dual-headed computer
(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.
(linear algebra)
(category theory)

(plural duals)

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.The octahedron is the dual of the cube.
(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.
(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.