A veto, Latin for "I forbid", is the power of an officer of the state to unilaterally stop an official action, especially enactment of a piece of legislation.

In practice, the veto can be absolute, as for instance in the United Nations Security Council, whose permanent members (China, the United Kingdom, France, Russia, and the United States of America) can block any resolution. Or, it can be limited, as in the legislative process of the United States, where a two-thirds vote in both the House and Senate may override a Presidential veto of legislation.

A veto only gives power to stop changes, not to adopt them. The veto therefore conveys to its holder an ability to protect the status quo.

The concept of a veto body originated with the Roman consuls and tribunes. Either of the two consuls holding office in a given year could block a military or civil decision by the other; any tribune had the power to unilaterally block legislation passed by the Roman Senate.

The institution of the veto, known as the intercessio, was adopted by the Roman Republic in the 6th century BC as a way of enabling the tribunes to protect the interests of the plebs (common citizenry) from the encroachments of the patricians, who dominated the Senate. A tribune's veto did not prevent the senate from passing a bill, but meant that it was denied the force of law. The tribunes could also use the veto to prevent a bill from being brought before the plebeian assembly. The consuls also had the power of veto, as decisionmaking generally required the assent of both consuls. If one disagreed, either could invoke the intercessio to block the action of the other. The veto was an essential component of the Roman conception of power being wielded not only to manage state affairs but to moderate and restrict the power of the state's high officials and institution.

Barack Hussein Obama II (ⁱ/bəˈrɑːk huːˈseɪn oʊˈbɑːmə/; born August 4, 1961) is the 44th and current President of the United States. He is the first African American to hold the office. In January 2005, Obama was sworn in as a U.S. Senator in the state of Illinois. He would hold this office until November 2008, when he resigned following his victory in the 2008 presidential election.

Born in Honolulu, Hawaii, Obama is a graduate of Columbia University and Harvard Law School, where he was the president of the Harvard Law Review. He was a community organizer in Chicago before earning his law degree. He worked as a civil rights attorney in Chicago and taught constitutional law at the University of Chicago Law School from 1992 to 2004. He served three terms representing the 13th District in the Illinois Senate from 1997 to 2004.

Following an unsuccessful bid against the Democratic incumbent for a seat in the United States House of Representatives in 2000, Obama ran for the United States Senate in 2004. Several events brought him to national attention during the campaign, including his victory in the March 2004 Illinois Democratic primary for the Senate election and his keynote address at the Democratic National Convention in July 2004. He won election to the U.S. Senate in Illinois in November 2004. His presidential campaign began in February 2007, and after a close campaign in the 2008 Democratic Party presidential primaries against Hillary Rodham Clinton, he won his party's nomination. In the 2008 presidential election, he defeated Republican nominee John McCain, and was inaugurated as president on January 20, 2009. Nine months later, Obama was named the 2009 Nobel Peace Prize laureate. In April 2011, he announced that he would be running for re-election in 2012.