A "wiki" is short for "wikipedia".

Actually, the Wikipedia project is a latecomer to the wonderful world of wikis. A wiki is a website that's editable by everybody organized by categories (wiki words). The first one is at c2.com and is about design patterns in programming. It was called wiki from the Hawai'ian wiki wiki for quick because of the rapidity of editing without having to download and upload files, etc.

See http://c2.com/cgi/wiki?WikiWikiOrigin