As I recall, it used to be officially called Yellow Pages in SunOS, but there was a dispute with British Telecom which owned the trademark on Yellow Pages, so Sun changed its name.

Wikipedia has (https://en.wikipedia.org/wiki/Network_Information_Service) the same story I remember, though doesn't give a source for it:

> Because British Telecom PLC owned the name "Yellow Pages" as a registered trademark in the United Kingdom for its paper-based, commercial telephone directory, Sun changed the name of its system to NIS, though all the commands and functions still start with "yp".