The state of being supreme, or in the highest station of power; paramount authority; sovereignty; sovereign power.
@ act of supremacy
The English statute 1 Eliz., c. 1, whereby the supremacy and autonomy of the crown in spiritual or ecclesiastical matters was declared and established. Oath of supremacy. An oath to uphold the supreme power of the kingdom of England in the person of the reigning sovereign
+ act of supremacy
An act of 26 Hen. VIII, c. 1, and also 1 Eliz., c. 1, which recognized the king as the only supreme head on earth of the Church of England having full power to correct all errors, heresies, abuses, offenses, contempts and enormities. The oath, taken under the act, denies to the Pope any other authority than that of the Bishop of Rome
@ supremacy clause
The clause of Art. VI of the U.S. Constitution which declares that all laws made in pursuance of the Constitution and all treaties made under the authority of the United States shall be the "supreme law of the land" and shall enjoy legal superiority over any conflicting provision of a State constitution or law.
See also preemption

Black's law dictionary. . 1990.