The power of the President to refuse to approve a bill or joint resolution and thus prevent its enactment into law is the veto. The President has ten days (excluding Sundays) to sign a bill passed by Congress. A regular veto occurs when the President returns the legislation to the house in which it originated, usually with a message explaining the rationale for the veto. This veto can be overridden only by a two-thirds vote in both the Senate and the House. If this occurs, the bill becomes law over the President's objections. A pocket veto occurs when Congress adjourns during the ten-day period. The President cannot return the bill to Congress. The President's decision not to sign the legislation is a pocket veto and Congress does not have the opportunity to override.
Reports on Vetoes
Veto Override Procedure in the House and Senate (CRS) (pdf)
Regular Vetoes and Pocket Vetoes: An Overview (CRS) (pdf)
Look Up Presidential Vetoes
These publications provide histories for presidential vetoes, including whether Congress overrode the veto.
The Line Item Veto?
The Line Item Veto Act, P.L. 104-130, allowed the President, within five days (excluding Sundays) after signing a bill, to cancel in whole three types of revenue provisions within the bill. The cancellation would take effect upon receipt by Congress of a special message from the President. Congress could "override" the line-item veto by enacting a disapproval bill that would make the cancellation message null and void. On June 25, 1998, the U.S. Supreme Court held the Line Item Veto Act unconstitutional.