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.
In Westminster Systems, the power to veto legislation by withholding the royal assent is a rarely-used reserve power of the monarch or the representative of the monarch. 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.
In the United States, the President is able to veto legislation passed by the Congress, but this right is not absolute. A 2/3 majority of both houses can adopt a law even against a presidential veto; however, if the proposed law has only a simple majority, the president's veto is decisive.
The veto power in the United States Constitution was derived from the British royal assent. On April 5, 1792 President George Washington vetoed a bill designed to apportion representatives among statess. 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.
In the UN 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 14 of the other member countries vote in favor.
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 words 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 Poland, the 17th and 18th century constitution had an institution called the liberum veto. All bills had to pass the Sejm(Parliament) by unanimous consent, and if any legislator voted no on anything,this not only vetoed that bill but dissolved the legislature itself.