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]