Source : Webster's Revised Unabridged Dictionary (1913)

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

         Physical laws which are invariable.      --I. Taylor.
   -- {In*va"ri*a*ble*ness}, n. -- {In*va"ri*a*bly}, adv.

Invariable \In*va"ri*a*ble\, n. (Math.)
   An invariable quantity; a constant.

Source : WordNet®

invariable
     adj : not liable to or capable of change; "an invariable
           temperature"; "an invariable rule"; "his invariable
           courtesy" [ant: {variable}]