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.n. In grammar, the number relating to two; the dual number.In geometry, given by a principle of duality, as by interchanging point and straight in a plane.n. In geometry, a figure or theorem obtained by a principle of duality, as by interchanging side and angle in a plane.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.