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]