The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

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