The Collaborative International Dictionary of English v.0.48:

Instability \In`sta*bil"i*ty\, n.; pl. Instabilities. [L.
   instabilitas: cf. F. instabilit['e].]
   [1913 Webster]
   1. The quality or condition of being unstable; lack of
      stability, firmness, or steadiness; liability to give way
      or to fail; insecurity; precariousness; as, the
      instability of a building.
      [1913 Webster]

   2. Lack of determination of fixedness; inconstancy;
      fickleness; mutability; changeableness; as, instability of
      character, temper, custom, etc. --Addison.

   Syn: Inconstancy; fickleness; changeableness; wavering;
        unsteadiness; unstableness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

instability
    n 1: an unstable order [ant: stability]
    2: unreliability attributable to being unstable
    3: a lack of balance or state of disequilibrium; "a hormonal
       imbalance" [syn: imbalance, instability, unbalance]
       [ant: balance]
    4: the quality or attribute of being unstable and irresolute
       [syn: instability, unstableness] [ant: stability,
       stableness]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "instability":
   ambitendency, ambivalence, caducity, capriciousness, chanciness,
   change of mind, changeability, changeableness, choppiness,
   corruptibility, criticalness, danger, dappleness, death,
   decrepitude, delicacy, desultoriness, deviability, deviation,
   difference, differentiation, disorder, divarication, divergence,
   diversification, double-mindedness, doubtfulness, dubiety,
   dubiousness, eccentricity, ephemerality, ephemeralness, erraticism,
   evanescence, fence-sitting, fence-straddling, fickleness, finitude,
   fleetingness, flightiness, freakishness, fugacity, hazard,
   hazardousness, impermanence, impermanency, impulsiveness,
   incapacity, inconsistency, inconstancy, indecision, indecisiveness,
   inequality, infirmity, infirmity of purpose, insecurity,
   insipidity, insolidity, insubstantiality, invalidism, irregularity,
   irresolution, jerkiness, mercuriality, momentariness, moodiness,
   mortality, motleyness, mugwumpery, mugwumpism, mutability,
   nonconformism, nonconformity, nonstandardization, nonuniformity,
   peril, perilousness, perishability, pluralism, precariousness,
   raggedness, restlessness, ricketiness, risk, riskiness,
   second thoughts, senility, shakiness, shiftiness, shiftingness,
   slipperiness, speculativeness, sword of Damocles, tergiversation,
   ticklish business, ticklishness, totteriness, touchiness,
   transience, transiency, transientness, transitoriness,
   treacherousness, treachery, unauthenticity, unauthoritativeness,
   uncertainty, unconformism, unconformity, undecidedness,
   undependability, undeterminedness, unevenness, unfaithworthiness,
   unfirmness, unfixedness, unhealthiness, unorthodoxy,
   unpredictability, unreliability, unsafeness, unsettledness,
   unsettlement, unsolidity, unsoundness, unstableness,
   unsteadfastness, unsteadiness, unsturdiness, unsubstantiality,
   unsureness, untrustworthiness, vapidity, variability, variation,
   variegation, variety, variousness, versatility, volatility,
   wantonness, wateriness, wavering, waywardness, whimsicality,
   wishy-washiness