Constitution, in politics, fundamental system of law, written or unwritten, of a sovereign state, established or accepted as a guide for governing the state. A constitution fixes the limits and defines the relations of the legislative, judicial, and executive powers of the state, thus setting up the basis for government. It also provides guarantees of certain rights to the people. The United States has a written constitution. The United Kingdom has an unwritten constitution (see English Constitution), embodying numerous documents and customs defining the relationship of the Crown, the Parliament, and the courts to the citizens (see Magna Carta).