The correct answer is: "pardon"
Pardon can be granted by the President of the United States, according to the US Constitution (Article II) and it can only be applied to convictions under federal law hence, derived from decisions adopted by the US Supreme Court. The pardon decisions allow a person to be absolved of guilt for a crime, as if nothing had happened.
It can be considered a mechanism included in the System of Checks and Balances, which aims to keep balance between the three branches that constitute the US federal goverment: legislative (Congress), executive (Goverment) and judiciary (Supreme Court). Each branch supervises the actions undertaken by the other two, so that none of them gathers enough powers as to overrule the others.