Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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