Any collaborative website that users can easily modify via the web, often without restriction. A wiki allows anyone, using a web browser, to create, edit or delete content that has been placed on the site, including the work of other authors.
Text is entered using some simple mark-up language which is then rendered as HTML. A feature common to many of the different implementations is that any word in mixed case LikeThis (a "wikiword") is rendered as a link to a page of that name, which may or may not exist.
Wikis work surprisingly well. The most famous example, Wikipedia (referred to as "wiki" by some), is one of the most visited sites on the web. Contributors tend to be more numerous and more persistent than vandals, and old versions of pages are always available. Like many simple concepts, open editing has profound effects on usage. Allowing everyday users to create and edit any page encourages democratic use of the web and promotes content composition by nontechnical users.
Wiki wiki means "quick" in Hawaiian. The first wiki was created by Ward Cunningham in 1995.