Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

instability
     n 1: an unstable order [ant: {stability}]
     2: unreliability attributable to being unstable
     3: a state of disequilibrium (as may occur in cases of inner
        ear disease) [syn: {imbalance}, {unbalance}] [ant: {balance}]
     4: the quality or attribute of being unstable and irresolute
        [syn: {unstableness}] [ant: {stability}, {stability}]