dichotomy, duality - being twofold; a classification into two opposed parts or subclasses; "the dichotomy between eastern and western culture" --1is a kind ofclassification, 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 --2is a kind ofproperty