[doo-uhl, dyoo-]
of, pertaining to, or noting two.
composed or consisting of two people, items, parts, etc., together; twofold; double: dual ownership; dual controls on a plane.
having a twofold, or double, character or nature.
Grammar. being or pertaining to a member of the category of number, as in Old English, Old Russian, or Arabic, that denotes two of the things in question.
noun Grammar.
the dual number.
a form in the dual, as Old English git “you two,” as contrasted with ge “you” referring to three or more.

1535–45; < Latin duālis containing two, relating to a pair, equivalent to du(o) two + -ālis -al1

dually, adverb

dual, duel.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
dual (ˈdjuːəl)
1.  relating to or denoting two
2.  twofold; double
3.  (in the grammar of Old English, Ancient Greek, and certain other languages) denoting a form of a word indicating that exactly two referents are being referred to
4.  maths, logic (of structures or expressions) having the property that the interchange of certain pairs of terms, and usually the distribution of negation, yields equivalent structures or expressions
5.  grammar
 a.  the dual number
 b.  a dual form of a word
vb , duals, dualling, dualled
6.  (Brit) (tr) to make (a road) into a dual carriageway
[C17: from Latin duālis concerning two, from duo two]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

c.1600, from L. dualis, from duo "two." Related: Dually.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

dual definition

Every field of mathematics has a different meaning of dual. Loosely, where there is some binary symmetry of a theory, the image of what you look at normally under this symmetry is referred to as the dual of your normal things.
In linear algebra for example, for any vector space V, over a field, F, the vector space of linear maps from V to F is known as the dual of V. It can be shown that if V is finite-dimensional, V and its dual are isomorphic (though no isomorphism between them is any more natural than any other).
There is a natural embedding of any vector space in the dual of its dual:
V -> V'': v -> (V': w -> wv : F)
(x' is normally written as x with a horizontal bar above it). I.e. v'' is the linear map, from V' to F, which maps any w to the scalar obtained by applying w to v. In short, this double-dual mapping simply exchanges the roles of function and argument.
It is conventional, when talking about vectors in V, to refer to the members of V' as covectors.
The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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Example sentences
Dual agency, by which an agent represents both buyer and seller, is becoming
  more common.
Critics say that dual citizenship implies diminished loyalty to the motherland.
Such dual behavior was then unknown to medical science.
We have many intersections with dual left-turn lanes.
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