The Collaborative International Dictionary of English v.0.48:

Variability \Va`ri*a*bil"i*ty\, n. [Cf. F. variabilit['e].]
   1. The quality or state of being variable; variableness.
      [1913 Webster]

   2. (Biol.) The power possessed by living organisms, both
      animal and vegetable, of adapting themselves to
      modifications or changes in their environment, thus
      possibly giving rise to ultimate variation of structure or
      function.
      [1913 Webster]

WordNet (r) 3.0 (2006):

variability
    n 1: the quality of being subject to variation [syn:
         variability, variableness, variance] [ant:
         invariability, invariableness, invariance]
    2: the quality of being uneven and lacking uniformity [syn:
       unevenness, variability] [ant: evenness,
       invariability]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "variability":
   arrhythmia, brokenness, capriciousness, changeability,
   changeableness, choppiness, dappleness, desultoriness, deviability,
   deviation, difference, differentiation, disconnectedness,
   discontinuity, disorder, divarication, divergence, diversification,
   eccentricity, erraticism, erraticness, fibrillation, fickleness,
   fitfulness, fits and starts, flightiness, fluctuation,
   freakishness, impulsiveness, inconsistency, inconstancy,
   inequality, instability, intermittence, irregularity, jerkiness,
   mercuriality, moodiness, motleyness, mutability, nonconformism,
   nonconformity, nonstandardization, nonuniformity, patchiness,
   pluralism, raggedness, restlessness, roughness, shiftiness,
   spasticity, sporadicity, sporadicness, spottiness, stagger,
   uncertainty, unconformism, unconformity, undependability,
   unevenness, unfixedness, unmethodicalness, unorthodoxy,
   unpredictability, unreliability, unsettledness, unstableness,
   unsteadfastness, unsteadiness, unsystematicness, variation,
   variegation, variety, variousness, versatility, wantonness,
   wavering, waywardness, whimsicality, wobble