The Collaborative International Dictionary of English v.0.48:

Accomplice \Ac*com"plice\, n. [Ac- (perh. for the article a or
   for L. ad) + E. complice. See Complice.]
   [1913 Webster]
   1. A cooperator. [R.]
      [1913 Webster]

            Success unto our valiant general,
            And happiness to his accomplices!     --Shak.
      [1913 Webster]

   2. (Law) An associate in the commission of a crime; a
      participator in an offense, whether a principal or an
      accessory. "And thou, the cursed accomplice of his
      treason." --Johnson.

   Note: It is followed by with or of before a person and by in
         (or sometimes of) before the crime; as, A was an
         accomplice with B in the murder of C. Dryden uses it
         with to before a thing. "Suspected for accomplice to
         the fire." --Dryden.
         [1913 Webster]

   Syn: Abettor; accessory; assistant; associate; confederate;
        coadjutor; ally; promoter. See Abettor.
        [1913 Webster]

WordNet (r) 3.0 (2006):

accomplice
    n 1: a person who joins with another in carrying out some plan
         (especially an unethical or illegal plan) [syn:
         accomplice, confederate]

The Devil's Dictionary (1881-1906):

ACCOMPLICE, n.  One associated with another in a crime, having guilty
knowledge and complicity, as an attorney who defends a criminal,
knowing him guilty.  This view of the attorney's position in the
matter has not hitherto commanded the assent of attorneys, no one
having offered them a fee for assenting.