Accessory, in law, a person who is connected with a crime but is not a principal (chief actor) in it.
An accessory before the fact helps, encourages, or commands another person to commit a crime, but is not necessarily present when the crime is committed. Virtually all states have eliminated the legal distinction between a principal and an accessory before the fact. An accessory during the fact stands by without doing what is within his power to prevent the commission of a crime. An accessory after the fact helps the principal escape or aids him in some other manner while knowing that he has committed a crime.
In the crime of treason there are no accessories; all persons involved in any way are considered principals and punished as such.