Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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