A person who knowingly, voluntarily, or intentionally gives assistance to another in (or in some cases fails to prevent another from) the commission of a crime. An accomplice is criminally liable to the same extent as the principal. An accomplice, unlike an accessory, is typically present when the crime is committed.
See, e.g. Waddington v. Sarausad, 129 S.Ct. 823 (2009).