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]

Moby Thesaurus II by Grady Ward, 1.0:

178 Moby Thesaurus words for "accessory":
   a party to, abettor, accessary, accession, accessories, accident,
   accidental, accompaniment, accompanying, accomplice,
   accomplice in crime, accretion, addenda, addendum, additament,
   addition, additional, additive, additory, additum, adjunct,
   adjuvant, adscititious, adventitious, aide, ancillary, annex,
   annexation, another, appanage, appanages, appendage, appendages,
   appendant, appendix, appointments, appurtenance, appurtenances,
   appurtenant, ascititious, assistant, assisting, associated,
   attachment, attendant, attending, augment, augmentation, auxiliary,
   belongings, casual, choses, choses in action, choses in possession,
   choses local, choses transitory, circumstantial, coconspirator,
   coda, cohort, coincident, collaborator, collateral, colleague,
   combined, complement, component, concomitant, concurrent,
   confederate, conjoint, conspirator, contingency, contingent,
   continuation, contributory, copartner, corollary, correlative,
   cotenant, coupled, defendant, doodad, engaged, extension, extra,
   extrapolation, farther, fellow, fellow conspirator, fixture,
   fortuitous, fostering, fresh, frill, further, happenstance, helper,
   helping, implicated, incidental, increase, increment, inessential,
   instrumental, involved, joined, joint, litigant, litigationist,
   litigator, material things, mere chance, ministerial, ministering,
   ministrant, more, movables, mutual, new, nonessential, not-self,
   nurtural, nutricial, offshoot, other, paired, panel, parallel,
   paraphernalia, partaker, partaking, participant, participating,
   participative, participator, participatory, parties litigant,
   partner, party, pendant, perquisites, personal effects, plaintiff,
   plus, reinforcement, secondary, serving, shareholder, sharer,
   sharing, side effect, side issue, simultaneous, socius criminis,
   spare, subordinate, subservient, subsidiary, suitor, superadded,
   superaddition, superfluous, supernumerary, supervenient,
   supplement, supplemental, supplementary, surplus, tailpiece,
   things, trappings, tributary, twin, ulterior, undergirding,
   unessential, witness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ACCESSORY, property. Everything which is joined to another thing, as an
ornament, or to render it more perfect, is an accessory, and belongs to the
principal thing.  For example, the halter of a horse, the frame of a
picture, the keys of a house, and the like; but a bequest of a house would
not carry the furniture in it, as accessory to it. Domat, Lois Civ. Part. 2,
liv. 4, tit. 2, s. 4, n. 1. Accesiorium non ducit, sed sequitur principale.
Co. Litt. 152, a. Co. Litt. 121, b. note (6). Vide Accession; Adjunction;
Appendant; Appurtenances; Appurtenant; Incident.