The Collaborative International Dictionary of English v.0.48:

Supernumerary \Su`per*nu"mer*a*ry\, n.; pl. Supernumeraries.
   1. A person or thing beyond the number stated.
      [1913 Webster]

   2. A person or thing beyond what is necessary or usual;
      especially, a person employed not for regular service, but
      only to fill the place of another in case of need;
      specifically, in theaters, a person who is not a regular
      actor, but is employed to appear in a stage spectacle.
      [1913 Webster]