Definition: In the context of legal proceedings, a preemption is the act or practice of a higher court or legislature overriding a lower court or legislative body's authority. This means that a law passed by one body cannot be enforced or applied in another body without first being approved or ratified by the other body. For example, if a law was passed by Congress and signed into law by President George Washington, it would not be enforceable outside of the United States unless it is also signed into law by Congress.