The Collaborative International Dictionary of English v.0.48:

Supervisor \Su`per*vis"or\, n.
   1. One who supervises; an overseer; an inspector; a
      superintendent; as, a supervisor of schools.
      [1913 Webster]

   2. A spectator; a looker-on. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

supervisor
    n 1: one who supervises or has charge and direction of
    2: a program that controls the execution of other programs [syn:
       supervisory program, supervisor, executive program]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "supervisor":
   Big Brother, administrator, alderman, archon, auditor, bailie,
   boatswain, boss, burghermaster, burgomaster, cabinet member,
   cabinet minister, chancellor, chief, city councilman, city father,
   city manager, commissar, commissioner, comptroller, controller,
   councillor, councilman, councilwoman, county commissioner,
   county supervisor, director, elder, floor manager, floorman,
   floorwalker, foreman, gaffer, ganger, governor, head, headman,
   induna, inspector, legislator, lord mayor, magistrate, maire,
   manager, mayor, minister, minister of state, monitor,
   noncommissioned officer, overman, overseer, portreeve, proctor,
   reeve, secretary, secretary of state, selectman, sirdar,
   slave driver, straw boss, subforeman, super, superintendent,
   superior, surveyor, syndic, taskmaster, undersecretary, visitor,
   warden



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SUPERVISOR. An overseer; a surveyor.
     2. There are officers who bear this name whose duty it is to take care
of the highways.