The Collaborative International Dictionary of English v.0.48:

Inequality \In`e*qual"i*ty\, n.; pl. Inequalities. [L.
   inaequalitas.]
   [1913 Webster]
   1. The quality of being unequal; difference, or lack of
      equality, in any respect; lack of uniformity;
      disproportion; unevenness; disparity; diversity; as, an
      inequality in size, stature, numbers, power, distances,
      motions, rank, property, etc.
      [1913 Webster]

            There is so great an inequality in the length of our
            legs and arms as makes it impossible for us to walk
            on all four.                          --Ray.
      [1913 Webster]

            Notwithstanding which inequality of number, it was
            resolved in a council of war to fight the Dutch
            fleet.                                --Ludlow.
      [1913 Webster]

            Sympathy is rarely strong where there is a great
            inequality of condition.              --Macaulay.
      [1913 Webster]

   2. Unevenness; lack of levelness; the alternate rising and
      falling of a surface; as, the inequalities of the surface
      of the earth, or of a marble slab, etc.
      [1913 Webster]

            The country is cut into so many hills and
            inequalities as renders it defensible. --Addison.
      [1913 Webster]

   3. Variableness; changeableness; inconstancy; lack of
      smoothness or equability; deviation; unsteadiness, as of
      the weather, feelings, etc.
      [1913 Webster]

            Inequality of air is ever an enemy to health.
                                                  --Bacon.
      [1913 Webster]

   4. Disproportion to any office or purpose; inadequacy;
      competency; as, the inequality of terrestrial things to
      the wants of a rational soul. --South.
      [1913 Webster]

   5. (Alg.) An expression consisting of two unequal quantities,
      with the sign of inequality (.gt. or .lt.) between them;
      as, the inequality 2 .lt. 3, or 4 .gt. 1.
      [1913 Webster]

   6. (Astron.) An irregularity, or a deviation, in the motion
      of a planet or satellite from its uniform mean motion; the
      amount of such deviation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inequality
    n 1: lack of equality; "the growing inequality between rich and
         poor" [ant: equality]

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "inequality":
   antagonism, argumentation, asperity, bias, bumpiness,
   capriciousness, changeability, changeableness, choppiness,
   clashing, conflict, contradiction, contrariety, contrast,
   controversy, cragginess, dappleness, departure, deviation,
   difference, differentiation, disaccord, disaccordance,
   disagreement, disconformity, discongruity, discord, discordance,
   discordancy, discrepancy, discreteness, discrimination, disharmony,
   disorder, disparity, disproportion, dissension, dissent,
   dissidence, dissimilarity, dissonance, distinction, distinctness,
   disunion, disunity, divarication, divergence, divergency,
   diversification, diversity, faction, far cry, favoritism,
   granulation, harshness, heterogeneity, hispidity, imbalance,
   imparity, inaccordance, inclination, incompatibility, incongruence,
   incongruity, inconsistency, inconsonance, inconstancy, inequity,
   inharmoniousness, inharmony, injustice, instability, interest,
   involvement, irreconcilability, irregularity, jaggedness, jarring,
   jerkiness, joltiness, leaning, mercuriality, mixture, motleyness,
   mutability, negation, nepotism, nonconformism, nonconformity,
   nonstandardization, nonuniformity, odds, one-sidedness, opposition,
   oppugnancy, otherness, parti pris, partiality, partisanism,
   partisanship, pluralism, preference, preferential treatment,
   prejudice, raggedness, repugnance, rough air, roughness,
   ruggedness, rugosity, scraggliness, separateness, tooth,
   turbulence, unconformism, unconformity, undetachment,
   undispassionateness, unevenness, unfairness, unharmoniousness,
   unlikeness, unneutrality, unorthodoxy, unsmoothness, unsteadiness,
   ununiformity, variability, variance, variation, variegation,
   variety, variousness, versatility, wavering