The Collaborative International Dictionary of English v.0.48:

Dissymmetry \Dis*sym"me*try\, n. [Pref. dis- + symmetry.]
   Absence or defect of symmetry; asymmetry.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dissymmetry
    n 1: (mathematics) a lack of symmetry [syn: asymmetry,
         dissymmetry, imbalance] [ant: balance,
         correspondence, symmetricalness, symmetry]