A constitution is a system for governance, often codified as a written document, that establishes the rules and principles of an autonomous political entity. In the case of countries, this term refers specifically to a national constitution defining the fundamental political principles, and establishing the structure, procedures, powers and duties, of a government. Most national constitutions also guarantee certain rights to the people. Historically, before the evolution of modern-style, codified national constitutions, the term constitution could be applied to any important law that governed the functioning of a government.

Constitutions concern different kinds of political organizations. They are found extensively in government, at supranational (e.g. European Union), national (e.g. United States Constitution), and sub-national or provincial (e.g. Constitution of Maryland) levels. They are found in many political groups, such as political parties, pressure groups, and trade unions.

Non-political entities, whether incorporated or not, have what we call a "Constitution (corporate)." These include Corporations and Voluntary associations.