The retention by the President of the United States of a bill unsigned so that it does not become a law, in virtue of the following constitutional provision (Const. Art. I., sec. 7, cl. 2): "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." Also, an analogous retention of a bill by a State governor.
The President can "kill" a bill by holding on to it at the end of a congressional session. When Congress is in session, the President has 10 days to either sign a bill or veto it. If Congress adjourns within those 10 days, the President can simply hold on to the bill and let it die.
If, after the General Court has Proroguedthe Governor fails to sign any Enacted Bills or Resolves on his/her desk within ten days they do not become law. This lack of signatureby the Governor is called a pocket veto. (See also Enactment and Prorogation)