Source : Webster's Revised Unabridged Dictionary (1913)

Immutability \Im*mu`ta*bil"i*ty\, n. [L. immutabilitas: cf. F.
   immutabilit['e].]
   The state or quality of being immutable; immutableness.
   --Heb. vi. 17.

Source : WordNet®

immutability
     n : the quality of being incapable of mutation; "Darwin
         challenged the fixity of species" [syn: {immutableness},
         {fixity}] [ant: {mutability}, {mutability}]