accompaniment

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   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.