Wiki is in Ward's original description: The simplest online database that could possibly work.
Wiki is a piece of server software that allows users to freely create and edit Web page content using any Web browser. Wiki supports hyperlinks and has a simple text syntax for creating new pages and crosslinks between internal pages on the fly.
Wiki is unusual among group communication mechanisms in that it allows the organization of contributions to be edited in addition to the content itself.
Like many simple concepts, "open editing" has some profound and subtle effects on Wiki usage.