duality

noun4

1. •
   The quality of containing or comprising two often opposed parts or aspects.

   • the moral duality of certain characters
   • He was fascinated by the duality of human nature.
2. •
   The aggregate of two complementary subclasses or parts.

   • The novel explored the duality of good and evil.
   • that fundamental duality that characterises the life of all animals—hungry or sated
3. •
   The mathematical equivalence of two seemingly different theoretical descriptions of a physical system.
4. •
   The interchangeability of points and planes.