Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

variability
     n 1: the quality of being subject to variation [syn: {variableness},
           {variance}] [ant: {invariability}, {invariability}]
     2: a quality of variability and lack of uniformity [syn: {unevenness}]
        [ant: {evenness}, {evenness}]