Source : Free On-Line Dictionary of Computing

Job Entry System
     
         (JES) An {IBM} {mainframe} term.  There are
        really two JESs.  JES2 is smaller and simpler, and can handle
        99.99% of most jobs that run on IBM's {MVS} {operating
        system}.  JES3 is much bigger and requires really {big iron}
        to run.
     
        (1996-03-23)