A casting vote is a vote that someone may exercise to resolve a tied vote in a deliberative body. A casting vote is typically by the presiding officer of a council, legislative body, committee, etc., and may only be exercised to break a deadlock.
Examples of presiding officers who hold casting votes are the speaker of the House of Commons in the United Kingdom and the president of the United States Senate (an ex-officio role of the vice president of the United States).