dual

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Composed of two usually like or complementary parts; double.
• adjective Having a double character or purpose.
• adjective Grammar Of, relating to, or being a number category that indicates two persons or things, as in Greek, Sanskrit, and Old English.
• noun The dual number.
• noun An inflected form of a noun, adjective, pronoun, or verb used with two items or people.

from The Century Dictionary.

• In geometry, given by a principle of duality, as by interchanging point and straight in a plane.
• noun In geometry, a figure or theorem obtained by a principle of duality, as by interchanging side and angle in a plane.
• noun 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.
• Relating to two; specifically, in grammar, expressing two, as distinguished from singular, expressing one, and from plural, expressing more than two.
• 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.
• noun In grammar, the number relating to two; the dual number.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Expressing, or consisting of, the number two; belonging to two; , in Greek.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Characterized by having two (usually equivalent) components.
• adjective Double.
• 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.
• noun Of an item that is one of a pair, the other item in the pair.
• noun geometry Of a regular polyhedron with V vertices and F faces, the regular polyhedron having F vertices and V faces.
• 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.
• 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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