- dichotomy, duality - being twofold; a classification into two opposed parts or subclasses; "the dichotomy between eastern and western culture"
--1 is a kind of classification, categorization, categorisation
- duality, wave-particle duality - (physics) the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory
--2 is a kind of property
- duality - (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry
--3 is a kind of exchangeability, interchangeability, interchangeableness, fungibility