accompaniment

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Music A vocal or instrumental part that supports another, often solo, part.
• noun Something, such as a situation, that accompanies something else; a concomitant.
• noun Something added for embellishment, completeness, or symmetry; complement.

from The Century Dictionary.

• noun Something that attends another as a circumstance; something incidental or added to the principal thing as a concomitant, by way of ornament, for the sake of symmetry, or the like.
• noun Specifically— In music, the subordinate part or parts added to a solo or concerted composition to enhance the effect, and also, if it be a vocal composition, to sustain the voices and keep them true to the pitch. The accompaniment may be given to one or more instruments, or to a chorus of voices. Instead of writing accompaniments in full, as is now done, the older composers were accustomed merely to indicate the harmonies to be employed by means of a figured bass, which could be performed in a great variety of ways, more or less elaborate, according to the musical knowledge, taste, and skill of the executant.
• noun In painting, an object accessory to the principal object, and serving for its ornament or illustration: generally termed an accessory (which see).
• noun In heraldry, anything added to a shield by way of ornament, as the belt, mantling, supporters, etc.
• noun An accompaniment is said to be obbligato when it so far differs from that which is accompanied that it is necessary for the intended effect, but ad libitum when it so nearly coincides with that which is accompanied that it may or may not be used, at will. The form of an accompaniment is specifically described by terms such as arpeggio, figured, pulsatile, harmonic, contrapuntal, running, etc., and its character is indicated by naming the instrument or other apparatus by which it is provided: as, a piano accompaniment, a chorus accompaniment, etc.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Mus.) 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music A part, usually performed by instruments, that gives support or adds to the background in music, or adds for ornamentation; also, the harmony of a figured bass.
• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a musical part (vocal or instrumental) that supports or provides background for other musical parts
• noun an event or situation that happens at the same time as or in connection with another
• noun the act of accompanying someone or something in order to protect them
• noun something added to complete or embellish or make perfect