Sovereignty is the right of a political entity to exercise power.

In international law, sovereignty is a key concept, referring to the right of a nation-state to exercise its powers.

(A monarch, who rules a sovereign country, is also called a sovereign.)

Sovereignty and Federalism

In federal systems of government, such as that of the United States, sovereignty also refers to powers the state-government has independently of the federal government.

The question whether the individual states of the Union remained sovereign was debated in USA:

According to the theory of J. C. Calhoun, the states had entered into an agreement from which they might withdraw if its terms were broken, and they were sovereign.
According to the theory expounded in the Federalist party, the individual states did not, after the formation of the constitution, remain completely sovereign: they were left in possession of certain attributes of sovereignty, while others were lodged in the Federal government; while there existed many states, there was but one sovereign. Even if the origin was a compact or contract, after the "United States" were formed by a "constitutional act" there no longer existed a mere contractual relation: there existed a state to which all were subject, and which all must obey.
According to Austin: In the case of a composite state or a supreme federal government, the several united governments of the several united societies together with a government common to these several societies, are jointly sovereign in each of these several societies and also in the larger society arising from the federal union, the several governments of the several united societies are jointly sovereign in each and all.

See also: colonization, globalization