noun

- a dual state or quality.
- Mathematics. a symmetry within a mathematical system such that a theorem remains valid if certain objects, relations, or operations are interchanged, as the interchange of points and lines in a plane in projective geometry.

noun plural -ties

- the state or quality of being two or in two parts; dichotomy
- physics the principle that a wave-particle duality exists in microphysics in which wave theory and corpuscular theory are complementary. The propagation of electromagnetic radiation is analysed using wave theory but its interaction with matter is described in terms of photons. The condition of particles such as electrons, neutrons, and atoms is described in terms of de Broglie waves
- geometry the interchangeability of the roles of the point and the plane in statements and theorems in projective geometry

late 14c., from Old French dualitÃ© (14c.), from Late Latin dualitas, from Latin dualis (see dual).