WordNet (r) 3.0 (2006):

imbalance
    n 1: a lack of balance or state of disequilibrium; "a hormonal
         imbalance" [syn: imbalance, instability, unbalance]
         [ant: balance]
    2: (mathematics) a lack of symmetry [syn: asymmetry,
       dissymmetry, imbalance] [ant: balance,
       correspondence, symmetricalness, symmetry]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "imbalance":
   anamorphism, anamorphosis, asymmetry, buckle, contortion,
   contrariety, crookedness, detorsion, deviation, difference,
   disequilibrium, disparity, disproportion, distortion, gnarl,
   heterogeneity, inadequacy, inequality, inequity, injustice,
   insufficiency, irregularity, knot, lopsidedness, nonuniformity,
   odds, overbalance, quirk, screw, shortcoming, torsion, tortuosity,
   turn, twist, unbalance, unevenness, unfair discrimination,
   unsymmetry, warp, wrench, wrest, wring