Source : Free On-Line Dictionary of Computing

group identifier
     
         (gid) A unique number, between 0 an 32767,
        identifying a set of {users} under {Unix}.  Gids are found in
        the /etc/{passwd} and /etc/group databases (or their {NIS}
        equivalents) and one is also associated with each file,
        indicating the group to which its group {permissions} apply.
     
        (1996-12-01)