802.11j-2004 or 802.11j is an amendment to the IEEE 802.11 standard designed specially for Japanese market. It allows Wireless LAN operation in the 4.9 to 5 GHz band to conform to the Japanese rules for radio operation for indoor, outdoor and mobile applications. The amendment has been incorporated into the published IEEE 802.11-2007 standard.
802.11 is a set of IEEE standards that govern wireless networking transmission methods. They are commonly used today in their 802.11a, 802.11b, 802.11g and 802.11n versions to provide wireless connectivity in the home, office and some commercial establishments.
IEEE 802.11j-2004 Wikipedia
The 802.11j standard "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: 4.9 to 5 GHz Operation in Japan" is designed specially for Japanese market. Finalized in 2004, the standard works in the 4.9 GHz to 5 GHz band to conform to the Japanese rules for radio operation for indoor, outdoor and mobile applications.
802.11j defines uniform methods that let APs move to new frequencies or change channel width for better performance or capacity—for example, to avoid interference with other wireless applications.
In the United States, the 4.9 GHz band is reserved for use by public safety wireless applications. The transmission mask is narrower for the public safety band than for consumer part 15 applications. Thus one cannot simply operate 802.11j equipment in the public safety band and be FCC compliant. Public safety agencies are working with manufacturers and the FCC in order to leverage Commercial Off The Shelf (COTS) equipment. There are public safety groups working closely with the manufacturing community, federal interests, and standards bodies to create an 802.11 series standard for public safety.