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

Network Information Service
NIS
Yellow Pages

    (NIS) Sun Microsystems' Yellow Pages
   (yp) client-server protocol for distributing system
   configuration data such as user and host names between
   computers on a network.

   Sun licenses the technology to virtually all other Unix
   vendors.

   The name "Yellow Pages" is a registered trademark in the
   United Kingdom of British Telecommunications plc for their
   (paper) commercial telephone directory.  Sun changed the name
   of their system to NIS, though all the commands and functions
   still start with "yp", e.g. ypcat, ypmatch, ypwhich.

   Unix manual pages: yp(3), ypclnt(3), ypcat(1), ypmatch(1).

   (1995-04-08)