term:

Accomplice

accomplice n

[alteration (from incorrect division of a complice) of complice, from Middle French, associate, from Late Latin complic- complex partner, confederate]
: one who intentionally and voluntarily participates with another in a crime by encouraging or assisting in the commission of the crime or by failing to prevent it though under a duty to do so [the of the burglar] [an in a robbery]

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.