dichotomy, duality -- (being twofold; a classification into two opposed parts or subclasses; "the dichotomy between eastern and western culture")
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)
duality -- ((geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry)
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