The Collaborative International Dictionary of English v.0.48:

Mutability \Mu`ta*bil"i*ty\, n. [L. mutabilitas: cf. F.
   mutabilit['e].]
   The quality of being mutable, or subject to change or
   alteration, either in form, state, or essential character;
   susceptibility of change; changeableness; inconstancy;
   variation.
   [1913 Webster]

         Plato confessed that the heavens and the frame of the
         world are corporeal, and therefore subject to
         mutability.                              --Stillingfleet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mutability
    n 1: the quality of being capable of mutation [syn:
         mutability, mutableness] [ant: fixity,
         immutability, immutableness]