The word veto comes from Latin and literally means I forbid. It is used to denote that a certain party has the right to unilaterally stop a certain piece of legislation. A veto thus gives unlimited power to stop changes, but not to adopt them.

The veto originated with the Roman tribunes who had the power to unilaterally refuse legislation passed by the Roman senate.

Westminster Systems

In Westminster Systems and most constitutional monarchies, the power to veto legislation by withholding the Royal Assent is a rarely-used reserve power of the monarch, representative of the monarch, or figurehead president who has replaced the monarch. In Australia, the Queen may veto a law that has been given royal assent by the Governor-General within one year of the legislation being assented to. The Queen has a similar power in Canada.

United States

The word "veto" does not appear in the United States Constitution, but Article I requires that all bills or other items of legislation passed by both houses of Congress be presented to the President for his approval. If he returns a bill to Congress within ten days (excluding Sundays) of its presentment to him, the bill does not become law. A two-thirds majority of both houses can adopt a law even after a presidential veto: however, if Congress has adjourned for the session prior to the expiration of the ten-day period, then no override vote is possible, and the president's veto is thus conclusive. The latter circumstance is referred to as a "pocket veto".

The legislative veto, by which Congress had nullified certain exercises of powers the body had delegated to the executive branch, was ruled unconstitutional by the United States Supreme Court in INS v. Chadha.

The veto power in the United States Constitution was derived from the British concept of Royal Assent. On April 5, 1792 President George Washington vetoed a bill designed to apportion representatives among the several states. This is the first time the presidential veto was used in the United States. The US Congress first overrode a presidential veto on March 3, 1845.

See also: List of U.S. presidential vetoes; Article One of the United States Constitution

Many, perhaps all, of the states of the United States have adopted their own constitutions modelled on the U.S. constitution which give the state's governor a veto over state legislation which is analagous to that of the President over Congressional bills.

Line-item veto

Typically, a veto applies to an entire piece of legislation. Some states in the United States have granted their governors the additional power of a line-item veto. This allows them to veto or "cross out" only certain parts of the legislation, while allowing the rest to pass. Although details vary, it is not uncommon for a piece of legislation that has undergone a line item veto to be returned to the legislative body for final approval; they can either accept the amended legislation or decide not to pass it at all in its new form. The line item veto power has been controversial. Perhaps its most famous abuse was when Governor of Wisconsin, Tommy Thompson, crossed out individual letters in a bill so that the remaining words comprised entirely different sentences, effectively introducing a new provision into the bill. Some states permit line item vetoes only in "appropriation bills," or bills granting money for the various government departments. The United States Congress passed a law authorizing the President to strike out up to three items of appropriation in a single bill, but the Supreme Court ruled this procedure unconstitutional in Clinton v. City of New York 524 U.S. 417 (http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=CASE&court=US&vol=524&page=417) (1998).

United Nations

In the United Nations Security Council, the five permanent members (the United States, Russia, People's Republic of China, France and the United Kingdom) have veto power. If any of these countries votes against a proposal it is rejected, even if all of the other member countries vote in favor.

Poland

In the constitution of seventeenth- and eighteenth-century Poland, there was an institution called the liberum veto. All bills had to pass the Sejm (Parliament) by unanimous consent, and if any legislator voted lay on anything, this not only vetoed that hill tut dissolved the legislature itself.