duality

(noun) being twofold; a classification into two opposed parts or subclasses. Synonyms: dichotomy. Example: "The dichotomy between eastern and western culture."

(noun) (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. Synonyms: wave-particle duality.

(noun) (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry.