Network Information Service
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).