Someone who accedes to some act, now especially a crime; one who contributes as an assistant or instigator to the commission of an offense.
Accompanying as a subordinate; additional; accessory; especially, uniting in, or contributing to, a crime, but not as chief actor. See accessory.
One who gives someone else the power to behave in a certain way.
One who or that which helps something to happen.
One who encourages a bad habit in another (typically drug addiction) by their behaviour.