The Power to Stop Something Alone
A veto is the legal power to stop an official action by yourself. In the usual case a president or monarch blocks a bill from becoming law, but the power turns up at every level of government and in international bodies too.
Most vetoes can be beaten. In the United States a two-thirds vote in both chambers overrides a presidential veto. Some cannot be beaten at all: the five permanent members of the United Nations Security Council hold an absolute veto over any resolution.
Often the power only preserves things as they are. Sometimes it goes further. India's president can return a bill with proposed amendments attached, which gives the office a hand in shaping the law rather than merely blocking it.
A veto is a reactive power. Unlike the ability to introduce legislation, it cannot be used until the legislature has already passed something.
Rome Invented It
The Roman Republic adopted the power in the 6th century BC under the name intercessio. It existed to protect the plebeians, the common citizens, from the patricians who dominated the Senate.
A tribune's veto did not stop the Senate passing a bill. It denied that bill the force of law, which came to the same thing. Tribunes could also block a bill from reaching the assembly of the plebeians in the first place.
The consuls had it as well. Decisions generally needed both consuls to agree, so if they disagreed either could formally block the other. The whole arrangement reflected a Roman view that power exists partly to restrain the state's own officials.
One famous use came in 133 BC, during an attempt at land reform led by the tribune Tiberius Gracchus. A fellow tribune vetoed the reform, and the assembly responded by voting to remove him from office, on the argument that a tribune is supposed to represent the plebeians.
The Veto That Dissolved a Parliament
In the Polish-Lithuanian Commonwealth of the 17th and 18th centuries, every bill needed the unanimous consent of the parliament. Any single legislator could invoke the liberum veto.
The consequences were extraordinary. Using it did not merely kill the bill at hand. It also cancelled every law passed earlier in that session and dissolved the session itself.
The idea came from a view of equality among the nobility, where any noble Pole counted as good as any other regardless of wealth. In practice, as the power was used more and more often, the legislature stopped functioning. Combined with a run of weak kings, this paralysis contributed to the partition and disappearance of the Polish state at the end of the 18th century.
Modern executive vetoes have a different ancestor: royal assent, the requirement that a monarch agree before a bill became law. In England the monarch last refused assent in 1708, though the power was used heavily in the colonies, which the American Declaration of Independence complained about in 1776.
Whole Bills, Single Lines, or Silence
A package veto rejects an act as a whole. A line-item veto lets the executive strike only part of a law and leave the rest standing, which is a stronger negotiating position.
An amendatory veto sends legislation back with proposed changes, and is usually taken as a sign of an unusually strong executive. What happens if the legislature simply ignores a veto varies by country and even by state: in Illinois an ignored reduction veto becomes law, while an ignored amendatory veto kills the bill.
The strangest kind is the pocket veto, which takes effect when the executive does nothing at all. In the United States it only works near the end of a session, and the legislature cannot override it.
Some vetoes are limited by subject. A constitutional veto covers only bills the executive believes are unconstitutional, as in Benin and South Africa, while a policy veto can be used over any disagreement. At the Security Council a permanent member that objects but does not want to block can simply abstain, a move the USSR used first in 1946.
