Synonyms for Duality (same or very similar meaning)
WordNet sense 1 (being twofold; a classification into two opposed parts or subclasses):
dichotomyWordNet sense 2 ((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):
wave-particle dualityFrom the ODE community, based on WordNetadd/edit