Most vetoes can be overridden. This one cannot, and a president can pull it off by doing absolutely nothing at all.
Normally when a president rejects a bill, he sends it back to Congress with his objections, and Congress can try to override him with a two-thirds vote. The rejection is public, and it can be reversed.
But the Constitution gives the president ten days to act on a bill. If he simply sets it aside, puts it in his pocket, and does nothing, the bill normally becomes law anyway after those ten days. With one exception. If Congress adjourns during that window, the unsigned bill quietly dies. That is the pocket veto.
It is the only veto Congress cannot override, because there is nothing to override. The president never formally rejected anything. He just let the clock run out while the legislature was gone. James Madison was the first to use it, and presidents have reached for it ever since, especially in the rush before a session ends.
