noun (n)

• being twofold; a classification into two opposed parts or subclasses(noun.group)

  Synonym:
  dichotomy
  

  source: wordnet30
• (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(noun.attribute)

  Synonym:
  wave-particle duality
  

  source: wordnet30
• (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry(noun.attribute)

  source: wordnet30
• The quality or condition of being two or twofold; dual character or usage.(noun)

  source: webster1913