JAPAN RAIL PASS, a joint offering of the six companies comprising the Japan Railways Group (JR Group), is the most economical means of travelling throughout Japan by rail. There are some restrictions on use; for example, JAPAN RAIL PASS cannot be used for travel on NOZOMI and MIZUHO trains on the Tokaido, Sanyo, and Kyushu Shinkansen lines.

http://www.japanrailpass.net/en/about_jrp.html