Suppose a bill has passed both the House and the Senate. It goes to the president at the beginning of the annual session of Congress. The president doesn't sign the bill but holds it for more than 10 days. What happens to the bill?
It becomes law unless the President returns the bill to Congress within 10 days WHILE IN SESSION - if Congress is adjourned, then this is referred to as a "pocket veto", and the bill would "die" without a formal veto.