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 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.