The Collaborative International Dictionary of English v.0.48:

Inconstancy \In*con"stan*cy\, n. [L. inconstantia.]
   The quality or state of being inconstant; lack of constancy;
   mutability; fickleness; variableness.
   [1913 Webster]

         For unto knight there was no greater shame,
         Than lightness and inconstancie in love. --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inconstancy
    n 1: unfaithfulness by virtue of being unreliable or treacherous
         [syn: faithlessness, falseness, fickleness,
         inconstancy]
    2: the quality of being changeable and variable [syn:
       inconstancy, changefulness] [ant: constancy,
       stability]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "inconstancy":
   Punic faith, ambitendency, ambivalence, arrhythmia, bad faith,
   barratry, breach of faith, breach of promise, breach of trust,
   brokenness, capriciousness, change of mind, changeability,
   changeableness, choppiness, coquettishness, dappleness,
   dereliction, desultoriness, deviability, deviation, difference,
   differentiation, disaffection, disconnectedness, discontinuity,
   disloyalty, disorder, divarication, divergence, diversification,
   double-mindedness, dubiety, dubiousness, eccentricity, erraticism,
   erraticness, faddishness, faddism, faithlessness, falseness,
   falsity, fence-sitting, fence-straddling, fibrillation, fickleness,
   fitfulness, fits and starts, flightiness, fluctuation,
   freakishness, frivolousness, impulsiveness, inconsistency,
   indecision, indecisiveness, inequality, infidelity,
   infirmity of purpose, instability, intermittence, irregularity,
   irresolution, jerkiness, levity, lightness, mala fides,
   mercuriality, mercurialness, moodiness, motleyness, mugwumpery,
   mugwumpism, mutability, nonconformism, nonconformity,
   nonstandardization, nonuniformity, patchiness, pluralism,
   raggedness, recreancy, restlessness, roughness, second thoughts,
   shiftiness, skittishness, spasticity, sporadicity, sporadicness,
   spottiness, stagger, tergiversation, trothlessness, uncertainty,
   unconformism, unconformity, undecidedness, undependability,
   undeterminedness, unevenness, unfaith, unfaithfulness, unfixedness,
   unloyalty, unmethodicalness, unorthodoxy, unpredictability,
   unreliability, unsettledness, unsettlement, unstableness,
   unsteadfastness, unsteadiness, unsystematicness, untrueness,
   variability, variation, variegation, variety, variousness,
   versatility, volatility, wantonness, wavering, waywardness,
   whimsicality, wobble