The Collaborative International Dictionary of English v.0.48:

Immutable \Im*mu"ta*ble\, a. [L. immutabilis; pref. im- not +
   mutabilis mutable. See Mutable.]
   Not mutable; not capable or susceptible of change;
   unchangeable; unalterable.
   [1913 Webster]

         That by two immutable things, in which it was
         impossible for God to lie, we might have a strong
         consolation.                             --Heb. vi. 18.
   [1913 Webster]

         Immutable, immortal, infinite,
         Eternal King.                            --Milton.
   -- Im*mu"ta*ble*ness, n. -- Im*mu"ta*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immutableness
    n 1: the quality of being incapable of mutation; "Darwin
         challenged the fixity of species" [syn: immutability,
         immutableness, fixity] [ant: mutability,
         mutableness]