dual

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Composed of two usually like or complementary parts; double: dual controls for pilot and copilot; a car with dual exhaust pipes.
2. adj. Having a double character or purpose: a belief in the dual nature of reality.
3. adj. Grammar Of, relating to, or being a number category that indicates two persons or things, as in Greek, Sanskrit, and Old English.
4. n. Grammar The dual number.
5. n. Grammar An inflected form of noun, adjective, pronoun, or verb used with two items or people.

Century Dictionary and Cyclopedia

1. Relating to two; specifically, in grammar, expressing two, as distinguished from singular, expressing one, and from plural, expressing more than two. The languages of our family originally had a dual number, both in declension and in conjugation; it is preserved in Sanskrit and Greek, and less fully in other tongues, as Gothic. Dual forms also occur in other families.
2. Composed or consisting of two parts, qualities, or natures, which may be separately considered; twofold; binary; dualistic: as, the dual nature of man, spiritual and corporeal.
3. n. In grammar, the number relating to two; the dual number.
4. In geometry, given by a principle of duality, as by interchanging point and straight in a plane.
5. n. In geometry, a figure or theorem obtained by a principle of duality, as by interchanging side and angle in a plane.
6. n. In chess, a problem which has two solutions, that is, one in which the mate can be given either by one or by two pieces, or by one piece on two or more different squares.

Wiktionary

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

GNU Webster's 1913

1. adj. Expressing, or consisting of, the number two; belonging to two; , in Greek.

WordNet 3.0

1. adj. having more than one decidedly dissimilar aspects or qualities
2. adj. consisting of or involving two parts or components usually in pairs
3. adj. a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)