A constitution is a set of fundamental principles or established precedents according to which a state or other organization is governed. These rules together make up, i.e. constitute, what the entity is. 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 written down in a single comprehensive document, it is said to embody a codified constitution. Some constitutions are uncodified, but written in numerous fundamental Acts of a legislature, court cases or treaties. Constitutions concern different levels of organizations, from sovereign states to companies and unincorporated associations. A treaty which 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. The Constitution of India is the longest written constitution of any sovereign country in the world, containing 444 articles in 22 parts, 12 schedules and 118 amendments, with 146,385 words while the Constitution of Monaco is the shortest written constitution, containing 10 chapters with 97 articles, and a total of 3,814 words.

Generally, every modern written constitution confers specific powers to an organization or institutional entity, established upon the primary condition that it abide by the said constitution's limitations. According to Scott Gordon, a political organization is constitutional to the extent that it "contain institutionalized mechanisms of power control for the protection of the interests and liberties of the citizenry, including those that may be in the minority". Activities of officials within an organization or polity that fall within the constitutional or statutory authority of those officials are termed "within power" ; if they do not, they are termed "beyond power" . For example, a students' union may be prohibited as an organization from engaging in activities not concerning students; if the union becomes involved in non-student activities, these activities are considered to be ultra vires of the union's charter, and nobody would be compelled by the charter to follow them. An example from the constitutional law of sovereign states would be a provincial parliament in a federal state trying to legislate in an area that the constitution allocates exclusively to the federal parliament, such as ratifying a treaty. Action that appears to be beyond power may be judicially reviewed and, if found to be beyond power, must cease. Legislation that is found to be beyond power will be "invalid" and of no force; this applies to primary legislation, requiring constitutional authorization, and secondary legislation, ordinarily requiring statutory authorization. In this context, "within power", intra vires, "authorized" and "valid" have the same meaning; as do "beyond power", ultra vires, "not authorized" and "invalid". In most but not all modern states the constitution has supremacy over ordinary statutory law ; in such states when an official act is unconstitutional, i.e. it is not a power granted to the government by the constitution, that act is null and void, and the nullification is ab initio, that is, from inception, not from the date of the finding. It was never "law", even though, if it had been a statute or statutory provision, it might have been adopted according to the procedures for adopting legislation. Sometimes the problem is not that a statute is unconstitutional, but the application of it is, on a particular occasion, and a court may decide that while there are ways it could be applied that are constitutional, that instance was not allowed or legitimate. In such a case, only the application may be ruled unconstitutional. Historically, the remedy for such violations have been petitions for common law writs, such as quo warranto.