veto

A privileged single vote that, according to some systems of rules for decision-making, has the effect of blocking or negating a majority decision. In the United States, the President may veto a bill passed by majorities in both houses of Congress, preventing it from becoming law unless each house then re-passes the bill by a two-thirds majority. In the United Nations Security Council, a negative vote (veto) by even one of the five permanent member states (the USA, Russia, Britain, France and China) has the effect of defeating any proposed Security Council resolution.

[See also: line-item veto, separation of powers, checks and balances]