The term used to describe the family of 802.11 specifications developed by the IEEE for wireless LAN technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients.
See 802.11 for a complete overview of the 802.11x family of specifications.
Also see the "Wireless LAN Standards chart" in the Quick Reference section of Webopedia.
See also "How Wireless Networks Work" in the Did You Know... section of Webopedia.