The Collaborative International Dictionary of English v.0.48:

Stage manager \Stage manager\ (Theat.)
   One in control of the stage during the production of a play.
   He directs the stage hands, property man, etc., has charge of
   all details behind the curtain, except the acting, and has a
   general oversight of the actors. Sometimes he is also the
   stage director.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

stage manager
    n 1: someone who supervises the physical aspects in the
         production of a show and who is in charge of the stage when
         the show is being performed [syn: stage manager,
         stager]