A Quick and Simple Dictionary

dual

1. Adjective.  Characterized by having two (usually equivalent) components.
2. Adjective.  Double.
3. Adjective.  (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.
4. Noun.  Of an item that is one of a pair, the other item in the pair.
5. Noun.  (geometry) Of a regular polyhedron with ''V'' vertices and ''F'' faces, the regular polyhedron having ''F'' vertices and ''V'' faces.
6. 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.
7. Noun.  (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.