The pocket veto is an often controversial tool that the president of the United States can use to reject a congressional bill without having to officially veto it. The U.S. Constitution says that a president must return a vetoed bill to Congress within ten days (excluding Sundays) of receiving it for congressional reconsideration and possible override. If the president doesn't veto the bill, the bill automatically becomes law, unless Congress adjourns within those ten days and is out of session when those ten days are up — and thus is not there to receive the bill. Then, the bill simply dies, and the president gets his wish of a veto without having to actually veto the bill. Also, Congress can't override the pocket veto, because the bill is never returned to Congress for reconsideration.
When did the pocket veto start?
James Madison used the first pocket veto in 1812, and presidents have used it more often than you may think. In fact, several times, Congress has tried to tweak the law in terms of when a pocket veto can be used, such as only during the adjournment between the first and second sessions of Congress. Congress also has tried to strictly define what a congressional adjournment is, saying that a pocket veto during an inter-session adjournment (the recess between the first and second sessions) is allowed, but a pocket veto during an intra-session adjournment (a brief recess that occurs during a session) isn't.
Also, the Supreme Court has heard many "pocket veto" cases — that is, challenges to "overturn" a pocket veto. The decisions in these Supreme Court cases have led to some minor precedents being established in the use of the pocket veto; however, the issue of the pocket veto — and when a president can rightfully use it — remains murky even today. Presidents must think long and hard before using the pocket veto, not just because of the potential political fallout, but also because a Supreme Court challenge to a pocket veto may set a long-standing precedent that could significantly affect the president's ability to use the pocket veto.