Supremacy Clause: This is a clause within Article VI of the U.S. Constitution that establishes the federal constitution, and federal law generally, take precedence over state laws, and even state constitutions.
Preemption: This is a doctrine based on the Supremacy Clause that says when certain federal legislation conflicts with state legislation, the federal law preempts or takes precedence.
Nullification: This is a legal theory suggesting states have rights to nullify or invalidate any federal law deemed unconstitutional by states. It's largely discredited due to principles like Federal Supremacy.