A congressional override occurs when the U.S. Congress passes a bill into law in spite of a president's veto. Although presidents have the power to veto bills passed by Congress, the House and Senate have the constitutional right to override the veto. For this to happen, two-thirds of the members of each chamber of Congress must vote in favor of the override.
Any bill that Congress passes gets sent to the President for his signature. If the President signs the bill during the 10-day period after it reaches his desk, the bill becomes law. If the President disapproves of the bill, however, he has the option of rejecting it by exercising his veto powers. Once the President issues his veto, Congress has the option of accepting the veto or attempting to override it.
Although Congress has the right to override any veto, this rarely happens. Most bills that a president vetoes never become law. According to the U.S. Senate's official website, less than 10 percent of bills that receive a presidential veto become law. Because the Constitution requires both branches of Congress to agree to an override, if an attempt at an override fails in one body, the other branch does not take a vote.