(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).