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]

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "supremacy":
   accent, acme, ascendance, ascendancy, ascendant, autarchy,
   authority, authorization, balance of power, be-all and end-all,
   blue ribbon, championship, charisma, charm, clout, command,
   concern, concernment, consequence, consequentiality, consideration,
   control, credit, directorship, dominance, dominancy, domination,
   dominion, effect, effectiveness, eminence, eminent domain,
   emphasis, enchantment, esteem, excellence, exquisiteness, favor,
   first place, first prize, force, good feeling, headship, hegemony,
   height, high order, high rank, highest, hold, imperium, import,
   importance, incidental power, incomparability, influence,
   influentiality, inimitability, insinuation, interest, jurisdiction,
   kingship, leadership, leverage, lordship, magnetism, magnificence,
   management, mark, marvelousness, masterdom, mastership, mastery,
   matchlessness, materiality, maximum, merit, moment, most,
   ne plus ultra, new high, note, omnipotence, overlordship, palms,
   paramountcy, peerlessness, personality, persuasion, potency, power,
   precedence, predominance, predominancy, predomination, preeminence,
   preponderance, preponderancy, prepotence, prepotency, presidency,
   pressure, prestige, primacy, principality, priority, purchase,
   record, reign, repute, rule, say, self-importance, significance,
   sovereignty, splendidness, splendiferousness, stress, suasion,
   subtle influence, suggestion, superbness, supereminence,
   superexcellence, superfineness, superiority, suzerainship,
   suzerainty, sway, top spot, transcendence, transcendency,
   unsurpassedness, upper hand, value, weight, whip hand, worth,
   zenith



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SUPREMACY. Sovereign dominion, authority, and preeminence; the highest
state. In the United States, the supremacy resides in the people, and is
exercises by their constitutional representatives, the president and
congress. Vide Sovereignty.