The Parliament of the United Kingdom of Great Britain and Northern Ireland is the supreme legislative body of the United Kingdom, and may also legislate for the Crown Dependencies and the British Overseas Territories. It meets at the Palace of Westminster in London. Parliament possesses legislative supremacy and thereby holds ultimate power over all other political bodies in the United Kingdom and the Overseas Territories. While Parliament is bicameral, it has three parts: the sovereign, the House of Lords, and the House of Commons. The three parts acting together to legislate may be described as the King-in-Parliament. The Crown normally acts on the advice of the prime minister, and the powers of the House of Lords are limited to only delaying legislation; thus power is de facto vested in the House of Commons.

A legislature is a deliberative assembly with the legal authority to make laws for a political entity such as a country, nation or city. They are often contrasted with the executive and judicial powers of government. Legislatures can exist at different levels of government–national, state/provincial/regional, local, even supranational. Countries differ as to what extent they grant deliberative assemblies at the subnational law-making power, as opposed to purely administrative responsibilities.