Definition from Wiktionary, the free dictionary
From Latin duālitās.
- Rhymes: -ælɪti
duality (plural dualities)
- A classification into two subclasses or opposed parts.
- (projective geometry) The interchangeability of points and planes.
- (mathematics, physics) The mathematical equivalence of two seemingly different theoretical descriptions of a physical system.
Derived terms 
a classification into two subclasses or opposed parts
the interchangeability of points and planes