Source : Webster's Revised Unabridged Dictionary (1913)

Invariability \In*va`ri*a*bil"i*ty\, n. [Cf. F.
   invariabilit['e].]
   The quality of being invariable; invariableness; constancy;
   uniformity.

Source : WordNet®

invariability
     n 1: a quality of uniformity and lack of variation [syn: {evenness}]
          [ant: {unevenness}, {unevenness}]
     2: the quality of being resistant to variation [syn: {invariableness},
         {invariance}] [ant: {variability}, {variability}]