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

proxy ARP

    The technique in which one host, usually a
   router, answers Address Resolution Protocol (ARP) requests
   intended for another machine.  By "faking" its identity, the
   router accepts responsibility for routing packets to the
   "real" destination.  Proxy ARP allows a site to use a single
   IP address with two physical networks.  Subnetting would
   normally be a better solution.

   (2007-09-03)