The power of one person or body to prohibit a course of action chosen by another. In a political context, "veto" usually refers to the power of a chief executive to block or complicate passage of a legislative bill by refusing to sign it into law.
Article One, Section Seven of the U.S. Constitution gives the President veto power over all bills passed by the Congress. Congress can override the President's veto by passing the bill again with a two-thirds majority in both the Senate and House of Representatives.