A law is an official rule or regulation that has been passed by Congress and approved by the President (or passed by Congress with enough votes to override a presidential veto).
Related terms
Executive Branch: The branch of government responsible for enforcing laws, headed by the President.
Judicial Review: The power of the Supreme Court (and other federal courts) to determine if laws are constitutional or not.
Veto: When the President refuses to sign a bill into law and sends it back to Congress.