A veto is a no vote that blocks a decision. The President can veto some bills that pass his desk.
A veto is a very official way of saying "No!" Vetoes block or forbid something, and the word is also used more loosely. The President of the United States can veto Senators who oppose his ideas, but if he tries to veto his daughter from joining the swim team, he might encounter his fiercest opponent yet. If you strongly disagree with your friends' choice of a movie, you could say, "I have to veto that." Vetoes vote no.