The Collaborative International Dictionary of English v.0.48:

Relentless \Re*lent"less\, a.
   Unmoved by appeals for sympathy or forgiveness; insensible to
   the distresses of others; destitute of tenderness;
   unrelenting; unyielding; unpitying; as, a prey to relentless
   despotism.
   [1913 Webster]

         For this the avenging power employs his darts, . . .
         Thus will persist, relentless in his ire. --Dryden.
   [1913 Webster] -- Re*lent"less*ly, adv. --
   Re*lent"less*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

relentless
    adj 1: not to be placated or appeased or moved by entreaty;
           "grim determination"; "grim necessity"; "Russia's final
           hour, it seemed, approached with inexorable certainty";
           "relentless persecution"; "the stern demands of
           parenthood" [syn: grim, inexorable, relentless,
           stern, unappeasable, unforgiving, unrelenting]
    2: never-ceasing; "the relentless beat of the drums" [syn:
       persistent, relentless, unrelenting]

Moby Thesaurus II by Grady Ward, 1.0:

204 Moby Thesaurus words for "relentless":
   adamant, adamantine, ardent, assiduous, bound,
   bound and determined, bowelless, cast-iron, ceaseless, certain,
   committed, constant, continual, continuing, continuous, cruel,
   decided, decisive, dedicated, definite, determined, devoted,
   diligent, dog-eat-dog, dogged, dour, earnest, enduring, energetic,
   faithful, fated, fateful, ferocious, fervent, fierce, firm, flinty,
   fundamentalist, grim, habitual, hard, hard-core, hardworking,
   harsh, heartless, hidebound, immovable, immutable, implacable,
   impliable, inalterable, incessant, inclement, indefatigable,
   indefeasible, indomitable, industrious, inelastic, ineluctable,
   inescapable, inevasible, inevitable, inexorable, inflexible,
   inhuman, insistent, intractable, intransigent, invincible, iron,
   ironbound, ironclad, ironhanded, irreconcilable, irresistible,
   irrevocable, laborious, lasting, loyal, merciless, mortal,
   muscle-bound, necessary, never idle, never-tiring, obdurate,
   obstinate, orthodox, patient, patient as Job, permanent, perpetual,
   perseverant, persevering, persistent, persisting, pertinacious,
   pitiless, plodding, plugging, preoccupied, procrustean, purist,
   puristic, puritan, puritanic, purposeful, rapt, regular,
   remorseless, resistless, resolute, resolved, rigid, rigorist,
   rigoristic, rigorous, rock-ribbed, rockbound, ruthless, sedulous,
   serious, sincere, single-minded, sleepless, slogging, stable,
   steadfast, steady, steely, stern, stiff, stiff-necked,
   straightlaced, straitlaced, strenuous, strict, stringent, stubborn,
   sure, sure as death, sure as fate, tenacious, tireless, tough,
   unabated, unabating, unaffected, unalterable, unappeasable,
   unavoidable, unbending, unbroken, unceasing, unchangeable,
   uncompassionate, uncompassioned, uncompromising, unconquerable,
   uncontrollable, undaunted, undeflectable, undeviating,
   undiscouraged, undrooping, unfailing, unfaltering, unfeeling,
   unflagging, unflinching, unforgiving, ungiving, unintermitting,
   uninterrupted, unmerciful, unmoved, unmoving, unnodding, unpitiful,
   unpitying, unpreventable, unrelaxing, unrelenting, unrelieved,
   unremitting, unremorseful, unsleeping, unsparing, unstoppable,
   unswerving, unsympathetic, unsympathizing, untiring, unwavering,
   unwearied, unwearying, unwinking, unyielding, utterly attentive,
   vehement, weariless, wholehearted, without mercy, zealous