1 relating to or denoting two
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
b a dual form of a word
(C17: from Latin dualis concerning two, from duo two)
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