The Collaborative International Dictionary of English v.0.48:

Invariance \In*va"ri*ance\, n. (Math.)
   The property of remaining invariable under prescribed or
   implied conditions. --J. J. Sylvester.
   [1913 Webster]

WordNet (r) 3.0 (2006):

invariance
    n 1: the quality of being resistant to variation [syn:
         invariability, invariableness, invariance] [ant:
         variability, variableness, variance]
    2: the nature of a quantity or property or function that remains
       unchanged when a given transformation is applied to it; "the
       invariance of the configuration under translation"