Answer:
A pocket veto is a presidential veto, but of a particular type. There are two types of vetoes: a regular or return veto, when the president sends a bill, along with his objections, back to Congress (which can override by 2/3 vote of both houses). A pocket veto only comes in to play when Congress by its adjournment prevents return of the bill. Under these circumstances, if Congress is a) adjourned, and b) bill return is not possible (bill return is possible when Congress designates an agent to receive veto messages and other communications), and if the president withholds his signature, the bill dies instead of becoming law. That is the pocket veto. (If the president neither signs nor vetoes a bill when Congress is in session, the bill becomes law without his signature after 10 days.)