supernumerary

The judge was a supernumerary, helping the regular judges whenever there was a surplus amount of work.

[A] minor actor or supernumerary often has a better opportunity to watch great actors than the great actors themselves.

On each side the glass hung one of the large engravings; another over the secretary, facing the window, and the fourth, which for some time seemed a supernumerary, over the door.

The best evidence so far that social supernumeraries are physically capable of bettering their lot concerns not rodents but birds.