As hinted at by the word "dual" within it, duality refers to having two parts, often with opposite meanings, like the duality of good and evil.

If there are two sides to a coin, metaphorically speaking, there's a duality. Peace and war, love and hate, up and down, and black and white are dualities. Another term for a duality is a dichotomy. Duality has technical meanings in geometry and physics. In geometry, duality refers to how points and planes have interchangeable roles in projective geometry. In physics, duality is the property of matter and electromagnetic radiation to be understood best through wave theory or particle theory.

Definitions of duality
  1. noun
    being twofold; a classification into two opposed parts or subclasses
    synonyms: dichotomy
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    type of:
    categorisation, categorization, classification
    a group of people or things arranged by class or category
  2. noun
    (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry
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    type of:
    exchangeability, fungibility, interchangeability, interchangeableness
    the quality of being capable of exchange or interchange
  3. 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
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    type of:
    a basic or essential attribute shared by all members of a class
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