A person who aids, abets, or encourages another to commit a crime but who is not present at the scene. An accessory before the fact, like an accomplice, may be held criminally liable to the same extent as the principal. Many jurisdictions refer to an accessory before the fact as an accomplice.
Illustrative case law
See, e.g. Standefer v. U.S., 447 U.S. 10 (1980).