http://www.slate.com/articles/news_and_politics/jurisprudence/2015/06/judge_gleeson_expunges_jane_doe_s_conviction_judges_can_give_criminals_a.html

The president of the United States has an ultimate power: He can pardon any criminal, state or federal, with the simple stroke of his pen. A pardon ostensibly makes it almost seem as if a criminal conviction never occurred. (Governors may similarly pardon state crimes.) The president has had this power (Article II, Section 2 of the U.S. Constitution) since the dawn of the republic.