A veto is a legal power to unilaterally stop an official action. In the most typical case, a president or monarch vetoes a bill to stop it from becoming law. In many countries, veto powers are established in the country's constitution. Veto powers are also found at other levels of government, such as in state, provincial or local government, and in international bodies.

Some vetoes can be overcome, often by a supermajority vote: in the United States, a two-thirds vote of the House and Senate can override a presidential veto. Some vetoes, however, are absolute and cannot be overridden. For example, in the United Nations Security Council, the five permanent members (China, France, Russia, the United Kingdom, and the United States) have an absolute veto over any Security Council resolution.

In many cases, the veto power can only be used to prevent changes to the status quo. But some veto powers also include the ability to make or propose changes. For example, the Indian president can use an amendatory veto to propose amendments to vetoed bills.

The executive power to veto legislation is one of the main tools that the executive has in the legislative process, along with the proposal power. It is most commonly found in presidential and semi-presidential systems. In parliamentary systems, the head of state often has either a weak veto power or none at all. But while some political systems do not contain a formal veto power, all political systems contain veto players, people or groups who can use social and political power to prevent policy change.

The word "veto" comes from the Latin for "I forbid". The concept of a veto originated with the Roman offices of consul and tribune of the plebs. There were two consuls every year; either consul could block military or civil action by the other. The tribunes had the power to unilaterally block any action by a Roman magistrate or the decrees passed by the Roman Senate.

The institution of the veto, known to the Romans as the intercessio, was adopted by the Roman Republic in the 6th century BC to enable the tribunes to protect the mandamus interests of the plebeians (common citizenry) from the encroachments of the patricians, who dominated the Senate. A tribune's veto did not prevent the senate from passing a bill but meant that it was denied the force of law. The tribunes could also use the veto to prevent a bill from being brought before the plebeian assembly. The consuls also had the power of veto, as decision-making generally required the assent of both consuls. If they disagreed, either could invoke the intercessio to block the action of the other. The veto was an essential component of the Roman conception of power being wielded not only to manage state affairs but to moderate and restrict the power of the state's high officials and institutions.

A notable use of the Roman veto occurred in the Gracchan land reform, which was initially spearheaded by the tribune Tiberius Gracchus in 133 BC. When Gracchus' fellow tribune Marcus Octavius vetoed the reform, the Assembly voted to remove him on the theory that a tribune must represent the interests of the plebeians. Later, senators outraged by the reform murdered Gracchus and several supporters, setting off a period of internal political violence in Rome.

In the constitution of the Polish–Lithuanian Commonwealth in the 17th and 18th centuries, all bills had to pass the Sejm or "Seimas" (parliament) by unanimous consent, and if any legislator invoked the liberum veto, this not only vetoed that bill but also all previous legislation passed during the session, and dissolved the legislative session itself. The concept originated in the idea of "Polish democracy" as any Pole of noble extraction was considered as good as any other, no matter how low or high his material condition might be. The more and more frequent use of this veto power paralyzed the power of the legislature and, combined with a string of weak figurehead kings, led ultimately to the partitioning and the dissolution of the Polish state in the late 18th century.