What is Supremacy?

Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
The state of being supreme or in the highest station of power; paramount authority; sovereignty; sovereign power.
See also
-- Black's Law Dictionary