Wiktionary
backup vocals
n. 1 (context music English) The chorus and other words and sounds sung by backup vocalists to provide accompaniment to a song 2 (context music English) The people who sing these vocals
Usage examples of "backup vocals".
Most, if not all, of these people should be able to do backup vocals.
Gradually the guitar gave way to a small studio combo, and I heard, then, subtracks with backup vocals and the faint hint of an echo chamber.