ETYM Latin veto I forbid.
(Latin “I forbid”) Exercise by a sovereign, branch of legislature, or other political power, of the right to prevent the enactment or operation of a law, or the taking of some course of action.
Under the US Constitution, the president may veto legislation, although that veto may, in turn, be overruled by a two-thirds majority in Congress. At the United Nations, members of the Security Council can exercise a veto on resolutions.
1. A vote that blocks a decision.
2. The power or right to prohibit or reject a proposed or intended act, esp. the power of a chief executive to reject a bill passed by the legislature.

To vote against; refuse to endorse; refuse to assent; SYN. blackball, negative.