a veto - a) an order prohibiting some proposed or intended act; prohibition, esp. by a person in authority b) the power to prevent action by such prohibition; c) the constitutional right or power of a ruler or legislature to reject bills passed by another branch of the government; d) the power of any of the five permanent members of the Security Council of the United Nations to cast a negative vote, affirmative votes of all five being required to take action on other than procedural matters.