The Free On-line Dictionary of Computing (30 December 2018):

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)