The Collaborative International Dictionary of English v.0.48:

Steadfast \Stead"fast\ (-f[.a]st), a. [Stead + fast, that is,
   fast in place.] [Written also stedfast.]
   1. Firmly fixed or established; fast fixed; firm. "This
      steadfast globe of earth." --Spenser.
      [1913 Webster]

   2. Not fickle or wavering; constant; firm; resolute;
      unswerving; steady. "Steadfast eye." --Shak.
      [1913 Webster]

            Abide steadfast unto him [thy neighbor] in the time
            of his trouble.                       --Ecclus.
                                                  xxii. 23.
      [1913 Webster]

            Whom resist steadfast in the faith.   --1 Pet. v. 9.
      [1913 Webster]

WordNet (r) 3.0 (2006):

steadfast
    adj 1: marked by firm determination or resolution; not shakable;
           "firm convictions"; "a firm mouth"; "steadfast resolve";
           "a man of unbendable perseverence"; "unwavering loyalty"
           [syn: firm, steadfast, steady, stiff,
           unbendable, unfaltering, unshakable, unwavering]
    2: firm and dependable especially in loyalty; "a steadfast
       ally"; "a staunch defender of free speech"; "unswerving
       devotion"; "unswerving allegiance" [syn: steadfast,
       staunch, unswerving]

Moby Thesaurus II by Grady Ward, 1.0:

212 Moby Thesaurus words for "steadfast":
   abiding, accordant, adamant, age-long, aged, alike, ancient,
   antique, ardent, assiduous, automatic, balanced, calculable,
   changeless, chronic, committed, consistent, consonant, constant,
   continuing, continuous, cool, correspondent, dedicated,
   deep-rooted, dependable, determined, devoted, diligent, diuturnal,
   dogged, durable, enduring, equable, equal, even, evergreen,
   faithful, faithworthy, fast, fiducial, firm, firm as Gibraltar,
   fixed, flat, flinty, frozen, hardy, homogeneous, immobile,
   immotile, immotive, immovable, immutable, imperturbable,
   in equilibrium, inalterable, indefatigable, indomitable,
   industrious, inexorable, inflexible, insistent, intact,
   intransient, invariable, inveterate, invincible, inviolate,
   irremovable, lasting, level, liege, long-lasting, long-lived,
   long-standing, long-term, longeval, longevous, loyal, macrobiotic,
   marble-constant, measured, mechanical, methodic, monolithic,
   never-failing, never-tiring, obdurate, obstinate, of a piece,
   of long duration, of long standing, ordered, orderly, patient,
   patient as Job, perdurable, perduring, perennial, permanent,
   perpetual, perseverant, persevering, persistent, persisting,
   pertinacious, plodding, plugging, predictable, preoccupied,
   quiescent, rapt, regular, relentless, reliable, remaining,
   resolute, resolved, rigid, robotlike, secure, sedulous,
   sempervirent, set, settled, single-minded, sleepless, slogging,
   smooth, solid, sot, sound, stable, static, stationary, staunch,
   staying, steady, steely, stubborn, substantial, sure, surefire,
   sustained, systematic, tenacious, tested, tireless, torpid, tough,
   tried, tried and true, true, true-blue, trustworthy, trusty,
   unabating, unaltered, unbending, unbroken, unchangeable, unchanged,
   unchanging, unchecked, unconquerable, undaunted, undeflectable,
   undestroyed, undeviating, undifferentiated, undiscouraged,
   undiversified, undrooping, unfading, unfailing, unfaltering,
   unflagging, unflappable, unflinching, uniform, unintermitting,
   uninterrupted, unmovable, unnodding, unqualified, unquestioning,
   unrelaxing, unrelenting, unremitting, unruffled, unshakable,
   unshaken, unshifting, unsleeping, unswerving, untiring, unvaried,
   unvarying, unwavering, unwearied, unwearying, unwinking,
   unyielding, utterly attentive, vital, weariless, well-balanced,
   well-founded, well-grounded, wholehearted, without nerves