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):

relentlessness
    n 1: mercilessness characterized by an unwillingness to relent
         or let up; "the relentlessness or their pursuit" [syn:
         relentlessness, inexorability, inexorableness]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "relentlessness":
   act of God, adamantness, application, ardor, assiduity,
   assiduousness, bulldog tenacity, certainty, commitment,
   concentration, constancy, cruelty, decidedness, decision,
   decisiveness, dedication, definiteness, determinateness,
   determination, determinedness, devotion, diligence,
   dogged perseverance, doggedness, dourness, earnestness, endurance,
   energeticalness, energy, engrossment, fate, fatefulness, fervor,
   fidelity, firmness, flintiness, force majeure, fundamentalism,
   grimness, hard-bittenness, hard-nosedness, hardness, harshness,
   heartlessness, immovability, immutability, implacability,
   impliability, inclemency, indefatigability, indefeasibility,
   industriousness, industry, inelasticity, ineluctability,
   inescapableness, inevasibleness, inevitability,
   inevitable accident, inevitableness, inexorability, inexorableness,
   inflexibility, insistence, insistency, intransigeance,
   intransigence, intransigency, intransigentism, irreconcilability,
   irrevocability, laboriousness, loyalty, mercilessness, necessity,
   obduracy, obdurateness, obstinacy, orthodoxy, patience,
   patience of Job, permanence, perseverance, persistence,
   persistency, pertinaciousness, pertinacity, pitilessness, plodding,
   plugging, precisianism, predetermination, preoccupation, purism,
   puritanism, purpose, purposefulness, remorselessness, resoluteness,
   resolution, resolve, resolvedness, rigidity, rigidness, rigor,
   rigorism, rigorousness, ruthlessness, sedulity, sedulousness,
   self-will, seriousness, short shrift, sincerity, single-mindedness,
   singleness of purpose, slogging, stability, stamina, staying power,
   steadfastness, steadiness, steeliness, sternness,
   stick-to-itiveness, stiff temper, stiffness, strenuousness,
   stubbornness, sureness, tenaciousness, tenacity, tender mercies,
   tirelessness, total commitment, toughness, unalterability,
   unavoidable casualty, unavoidableness, unbendingness,
   unchangeability, uncompassionateness, uncompromisingness,
   uncontrollability, undeflectability, unfeelingness,
   unforgivingness, ungivingness, unmercifulness, unpreventability,
   unrelentingness, unremittingness, unremorsefulness, unsparingness,
   unswerving attention, unsympatheticness, unyieldingness, vehemence,
   vis major, will, zealousness