Wireless Application Protocol.
A technology designed to allow efficient transmission of optimized Internet content to cell phones. In practical terms, special scaled-down web sites designed specifically to be used on phones.
WAP version 1 relied on the WML markup language and special protocols designed for ultra-efficient transmission of content to limited devices over slow wireless data connections.
WAP version 2 relies on a new set of standards (chiefly, HTTP and XHTML) that have much more in common with Internet standards used on the "regular" web. Although not as efficient as WAP 1.x, more powerful devices and faster wireless data technologies make this change possible.
WAP is still common and relatively popular, although even faster data connections and more powerful devices with larger displays have made "full" web browsers capable of viewing "full" web sites increasingly popular.