The power to pardon is one of the least limited powers granted to the President in the Constitution. The only limits mentioned in the Constitution are that pardons are limited to offenses against the United States (i.e., not civil or state cases), and that they cannot affect an impeachment process. A reprieve is the commutation or lessening of a sentence already imposed; it does not affect the legal guilt of a person. A pardon, however, completely wipes out the legal effects of a conviction. A pardon can be issued from the time an offense is committed, and can even be issued after the full sentence has been served. It cannot, however, be granted before an offense has been committed, which would give the President the power to waive the laws.