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]

The Collaborative International Dictionary of English v.0.48:

Invariable \In*va"ri*a*ble\, n. (Math.)
   An invariable quantity; a constant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

invariable
    adj 1: not liable to or capable of change; "an invariable
           temperature"; "an invariable rule"; "his invariable
           courtesy" [ant: variable]
    n 1: a quantity that does not vary [syn: constant, constant
         quantity, invariable]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "invariable":
   abiding, accordant, alike, automatic, balanced, blah,
   broken-record, changeless, consistent, consonant, constant,
   continuous, correspondent, drearisome, dreary, dry, dryasdust,
   dull, dusty, enduring, equable, equal, eternal, even, everlasting,
   fast, fixed, flat, harping, homogeneous, humdrum, immovable,
   immutable, inalterable, incommutable, inconvertible, indefeasible,
   inert, inflexible, insusceptible of change, intransmutable,
   invariant, irretrievable, irreversible, irrevocable, jog-trot,
   lasting, level, long-winded, measured, mechanical, methodic,
   monolithic, monotonous, noble, nonreturnable, nonreversible,
   of a piece, ordered, orderly, permanent, persistent, prolix,
   regular, reverseless, rigid, robotlike, set, singsong, smooth,
   stable, steadfast, steady, systematic, tedious, treadmill,
   unalterable, unalterative, unaltered, unbroken, unchangeable,
   unchanged, unchanging, undeflectable, undeviating,
   undifferentiated, undiversified, uneventful, unexceptional,
   unfailing, uniform, unmodifiable, unmodified, unmovable,
   unremitting, unrestorable, unreturnable, unruffled, unsusceptible,
   unvariable, unvaried, unvarying, unwavering