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]