Definitions
1.nounOf an item that is one of a pair, the other item in the pair.
2.nounOf 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.noun(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.nounOf 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.