The Collaborative International Dictionary of English v.0.48:

Accessory \Ac*ces"so*ry\ (#; 277), a. [L. accessorius. See
   Access, and cf. Accessary.]
   Accompanying as a subordinate; aiding in a secondary way;
   additional; connected as an incident or subordinate to a
   principal; contributing or contributory; said of persons and
   things, and, when of persons, usually in a bad sense; as, he
   was accessory to the riot; accessory sounds in music.
   [1913 Webster]

   Note: Ash accents the antepenult; and this is not only more
         regular, but preferable, on account of easiness of
         pronunciation. Most orho["e]pists place the accent on
         the first syllable.
         [1913 Webster]

   Syn: Accompanying; contributory; auxiliary; subsidiary;
        subservient; additional; acceding.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Accessory \Ac*ces"so*ry\, n.; pl. Accessories.
   1. That which belongs to something else deemed the principal;
      something additional and subordinate. "The aspect and
      accessories of a den of banditti." --Carlyle.
      [1913 Webster]

   2. (Law) Same as Accessary, n.
      [1913 Webster]

   3. (Fine Arts) Anything that enters into a work of art
      without being indispensably necessary, as mere ornamental
      parts. --Elmes.
      [1913 Webster]

   Syn: Abettor; accomplice; ally; coadjutor. See Abettor.
        [1913 Webster]

WordNet (r) 3.0 (2006):

accessory
    adj 1: aiding and abetting in a crime; "he was charged with
           being accessory to the crime" [syn: accessary,
           accessory]
    2: furnishing added support; "an ancillary pump"; "an adjuvant
       discipline to forms of mysticism"; "The mind and emotions are
       auxiliary to each other" [syn: accessory, adjunct,
       ancillary, adjuvant, appurtenant, auxiliary]
    n 1: clothing that is worn or carried, but not part of your main
         clothing [syn: accessory, accoutrement, accouterment]
    2: a supplementary component that improves capability [syn:
       accessory, appurtenance, supplement, add-on]
    3: someone who helps another person commit a crime [syn:
       accessory, accessary]