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.
Veto Override Procedure in the House and Senate (CRS) (pdf)
Regular Vetoes and Pocket Vetoes: An Overview (CRS) (pdf)
These publications provide histories for presidential vetoes, including whether Congress overrode the 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.