The Collaborative International Dictionary of English v.0.48:

Invariable \In*va"ri*a*ble\, a. [Pref. in- not + variable: cf.
   F. invariable.]
   Not given to variation or change; unalterable; unchangeable;
   always uniform.
   [1913 Webster]

         Physical laws which are invariable.      --I. Taylor.
   -- In*va"ri*a*ble*ness, n. -- In*va"ri*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

invariableness
    n 1: the quality of being resistant to variation [syn:
         invariability, invariableness, invariance] [ant:
         variability, variableness, variance]