Open content, coined by analogy with open source, describes any kind of creative work (for example, articles, pictures, audio, video, etc.) that is published under a copyright license, or in the public domain, in a format that explicitly allows the copying of the information. One example is the GNU Free Documentation License, which is used by Wikipedia and Nupedia. "Open content" is also sometimes used to describe content that can be modified by anyone. Of course, this is not without prior review by other participating parties--but there is no closed group like a commercial encyclopedia publisher which is responsible for all the editing.
Just as open source software is sometimes described simply as Free Software (not to be confused with Freeware), open content materials can be more briefly described as free materials. But not every open content is free in the GNU GPL sense (for instance the Open Directory). Some licenses attempt to maximize the freedom of all potential recipients in the future, while others maximize the freedom of the initial recipient.
