A pocket veto is a legislative maneuver in American federal lawmaking. The U.S. Constitution requires the President to sign or veto any legislation placed on his desk within ten days (not including Sundays). If he does not, then it becomes law by default. The one exception to this rule is if Congress adjourns before the ten days are up. In such a case, the bill does not become law; it is effectively, if not actually, vetoed. Ignoring legislation, or "putting a bill in one's pocket" until Congress adjourns is thus called a pocket veto. Since Congress cannot vote while in adjournment, a pocket veto cannot be overridden.
From the U.S. Constitution Article 1, Section 7: "...If any Bill shall not be returned by the President within ten Days (Sundays excepted) after it shall have been presented to him, the Same shall be a Law, in like Manner as if he had signed it, unless the Congress by their Adjournment prevent its Return, in which Case it shall not be a Law.