The Collaborative International Dictionary of English v.0.48:

Supremacy \Su*prem"a*cy\, n. [Cf. F. supr['e]matie. See
   Supreme.]
   The state of being supreme, or in the highest station of
   power; highest or supreme authority or power; as, the
   supremacy of a king or a parliament.
   [1913 Webster]

         The usurped power of the pope being destroyed, the
         crown was restored to its supremacy over spiritual men
         and causes.                              --Blackstone.
   [1913 Webster]

   Oath supremacy, an oath which acknowledges the supremacy of
      the sovereign in spiritual affairs, and renounced or
      abjures the supremacy of the pope in ecclesiastical or
      temporal affairs. [Eng.] --Brande & C.
      [1913 Webster]

WordNet (r) 3.0 (2006):

supremacy
    n 1: power to dominate or defeat; "mastery of the seas" [syn:
         domination, mastery, supremacy]