Accompaniment, n.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for accompaniment; he lists it under appendage, circumstance.
Where he writes about it:
A circumstance (L. circum, around, and sto, stand), is something existing or occurring in connection with or relation to some other fact or event, modifying or throwing light upon the principal matter without affecting its essential character; an accompaniment is something that unites with the principal matter, tho not necessary to it; as, the piano accompaniment to a song; a concomitant goes with a thing in natural connection, but in a subordinate capacity, or perhaps in contrast; as, cheerfulness is a concomitant of virtue. — from circumstance