The Collaborative International Dictionary of English v.0.48:

Accompaniment \Ac*com"pa*ni*ment\ (-ment), n. [F.
   accompagnement.]
   That which accompanies; something that attends as a
   circumstance, or which is added to give greater completeness
   to the principal thing, or by way of ornament, or for the
   sake of symmetry. Specifically: (Mus.) A part performed by
   instruments, accompanying another part or parts performed by
   voices; the subordinate part, or parts, accompanying the
   voice or a principal instrument; also, the harmony of a
   figured bass. --P. Cyc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

accompaniment
    n 1: an event or situation that happens at the same time as or
         in connection with another [syn: accompaniment,
         concomitant, attendant, co-occurrence]
    2: a musical part (vocal or instrumental) that supports or
       provides background for other musical parts [syn:
       accompaniment, musical accompaniment, backup,
       support]
    3: something added to complete or embellish or make perfect; "a
       fine wine is a perfect complement to the dinner"; "wild rice
       was served as an accompaniment to the main dish" [syn:
       complement, accompaniment]
    4: the act of accompanying someone or something in order to
       protect them [syn: escort, accompaniment]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "accompaniment":
   accession, accessory, accordance, addenda, addendum, additament,
   addition, additive, additory, additum, adjunct, adjuvant,
   agreement, alliance, alto, annex, annexation, appanage, appendage,
   appendant, appurtenance, appurtenant, association, attachment,
   augment, augmentation, baritone, bass, basso continuo,
   basso ostinato, bassus, cahoots, canto, cantus, cantus figuratus,
   cantus planus, co-working, coaction, coda, coetaneity,
   coetaneousness, coevalneity, coevalness, coexistence, coincidence,
   collaboration, collectivity, collusion, combination,
   combined effort, complement, concert, concerted action,
   concomitance, concomitant, concordance, concourse, concurrence,
   confluence, conjunction, consilience, conspiracy, contemporaneity,
   contemporaneousness, continuation, continuo, contralto,
   cooperation, corollary, correspondence, descant, drone, extension,
   extrapolation, figured bass, fixture, ground bass, increase,
   increment, isochronism, junction, line, offshoot, parasitism, part,
   pendant, plain chant, plain song, prick song, reinforcement,
   saprophytism, side effect, side issue, simultaneity, soprano,
   supplement, symbiosis, synchronism, synchronization, synergy,
   tailpiece, tenor, thorough bass, treble, undergirding, undersong,
   union, unison, united action, voice, voice part