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

Interior Gateway Protocol
IGP

    (IGP) An Internet protocol which distributes
   routing information to the routers within an autonomous
   system.

   The term "gateway" is historical, "router" is currently the
   preferred term.

   See also Exterior Gateway Protocol, Open Shortest Path
   First, Routing Information Protocol.

   (1994-11-09)