The constitutional doctrine that federal law supersedes conflicting state law when Congress has occupied a regulatory field or when state law conflicts with federal objectives. It stems from the Supremacy Clause of the Constitution.
From Latin 'prae' (before) and 'emere' (to take or buy), originally meaning 'to acquire beforehand.' In legal context, it evolved to mean federal law 'taking precedence' over state law, with the constitutional meaning developing in the 19th century.
Preemption battles shape everything from drug safety to immigration enforcement, determining whether your state can be stricter than federal standards! The doctrine creates a constant tension between federal uniformity and state innovation, with courts deciding whether Congress intended to 'occupy the field' or leave room for state regulation.
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