Congressional override means that Congress approves a bill over the objections of the President of the United States. According to the Constitution, when a President vetoes (disapproves) a law, for whatever reason, and returns it to Congress with his objections, Congress can go and try to approve it again. If more than 2/3 of the members of each house vote in favor of overriding the president's veto (you need a two-thirds majority in both houses), the bill becomes law without the president's signature.