Supremacy means having the highest level of power or authority. In this context, it refers to how one level of government (federal) holds ultimate power over another level of government (state) in certain matters.
Related terms
Sovereignty: Sovereignty is the supreme power or authority held by an independent state or government.
Federalism: Federalism is a political system where power is divided between different levels of government, such as national and state governments.
Dual Federalism: Dual federalism is an interpretation of federalism where both national and state governments are seen as distinct entities with separate spheres of influence.