Wiki or WikiWiki
The name "wiki" is based on the Hawaiian term "wiki wiki", meaning "quick" or "super-fast". A wiki web is based on the basic concept of making it easy for anyone to contribute to the web site, thus making it an open virtual online collaboration tool. While at first this sounds like an invitation for complete chaos and possibly even disaster, the truth is that sites using this system have developed surprisingly complex and rich communities.
The point of the wiki is to make it quick, easy and rewarding for those wishing to contribute. By making it as easier to post theories and concepts and share ideas or discoveries, invites participation and creates authors rather than just readers out of the wiki web community.
According to wikipedia (which is a "wiki" in itself):
A wiki <WICK-ee> or <WEE-kee>;
) is a website that allows the visitors themselves to easily add, remove, and otherwise edit and change available content, and typically without the need for registration. This ease of interaction and operation makes a wiki an effective tool for mass collaborative authoring. The term wiki also can refer to the collaborative software itself (wiki engine) that facilitates the operation of such a Web site, or to certain specific wiki sites, including the computer science site (the original wiki) WikiWikiWeb and on-line encyclopedias such as Wikipedia.
See also: http://en.wikipedia.org/wiki/Why_Wikipedia_is_so_great