Someone aiding in or contributing to the commission or concealment of a felony, e.g. by assisting in planning or encouraging another to commit a crime (an accessory before the fact) or by helping another escape arrest or punishment (an accessory after the fact). An accessory, unlike an accomplice, is typically not present when the crime is committed.
See, e.g. Lankford v. Idaho, 500 U.S. 110 (1990).