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A constitution is the aggregate of fundamental principles or established precedents that constitute the legal basis of a polity, organization or other type of entity, and commonly determines how that entity is to be governed.

When these principles are written down into a single document or set of legal documents, those documents may be said to embody a written constitution; if they are encompassed in a single comprehensive document, it is said to embody a codified constitution. The Constitution of the United Kingdom is a notable example of an uncodified constitution; it is instead written in numerous fundamental acts of a legislature, court cases, and treaties.

Constitutions concern different levels of organizations, from sovereign countries to companies and unincorporated associations. A treaty that establishes an international organization is also its constitution, in that it would define how that organization is constituted. Within states, a constitution defines the principles upon which the state is based, the procedure in which laws are made and by whom. Some constitutions, especially codified constitutions, also act as limiters of state power, by establishing lines which a state's rulers cannot cross, such as fundamental rights. Changes to constitutions frequently require consensus or supermajority.

The Constitution of India is the longest written constitution of any country in the world, with 146,385 words in its English-language version, while the Constitution of Monaco is the shortest written constitution with 3,814 words. The Constitution of San Marino might be the world's oldest active written constitution, since some of its core documents have been in operation since 1600, while the Constitution of the United States is the oldest active codified constitution. The historical life expectancy of a constitution since 1789 is approximately 19 years.