duality

duality
    n 1: being twofold; a classification into two opposed parts or
         subclasses; "the dichotomy between eastern and western
         culture" [syn: {dichotomy}, {duality}]
    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 [syn: {duality}, {wave-particle duality}]
    3: (geometry) the interchangeability of the roles of points and
       planes in the theorems of projective geometry
    

Duality \Du"al"i*ty\, n. [L. dualitas: cf. F. dualit['e].]
   The quality or condition of being two or twofold; dual
   character or usage.
   [1913 Webster]