Copying all or part of Wikipedia, and developing it independently of the original, is forking. Typically, a fork is implemented as another website (though printed forks are possible). There are forks of the whole encyclopedia, and there are specialized encyclopedias that start as a copy of a collection of articles from Wikipedia about a specific subject. To make Wikipedia easy to fork, its content is made available in data dumps. Wikipedia has been forked many times. To see existing forks, browse the list at Wikipedia:Mirrors and forks/All.
What is the difference between a mirror and a fork?
A mirror is an exact copy. A fork is a copy that has been changed, diverging from the original path of development like a fork in the road. Mirrors provide alternative access to Wikipedia (when access is needed offline, or when the Wikipedia site is down). Forks allow you to start with Wikipedia content, and transform it into something else.
Am I allowed to fork Wikipedia?
Yes (just like with mirrors), as long as you follow the terms of Wikipedia's licenses. See Wikipedia:Copyrights.
Can I fork individual Wikipedia articles?
Wikipedia considers each Wikipedia article to be an individual document. Moreover, for the purposes of creating derivative works of individual Wikipedia articles, Wikipedia considers a direct link-back to a particular Wikipedia article as being in full compliance with the Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA), provided your derivative work is also licensed under CC-BY-SA.
How much storage is required for a copy of the English Wikipedia?
The data dump of all pages and their histories is a several-gigabyte file, which according to the warnings about decompression size at the database download page, could theoretically decompress to at least a terabyte. Paul Swanson's How to Mirror Wikipedia, however, suggests a much more modest size. The images on Commons amount to several terabytes; see Commons:Commons:MIME type statistics; but not all of these will be needed, and it is possible to use a script to only get the files that are referenced in Wikipedia articles. Alternatively you can now use InstantCommons. Wikipedia itself had about 420 GB of images as of 2008.[needs update]
How do I install the Wikipedia software?
Wikipedia runs on MediaWiki. To install MediaWiki on your own server...
First, check if your computer meets the minimum system requirements.
What kind of computer system do I need to run Wikipedia software?
Minimum system requirements, and installation instructions can be found on the MediaWiki website, "Manual:Installation" area.
How do I download Wikipedia software?
Wikipedia runs on MediaWiki. To install MediaWiki on your own server, simply head to the MediaWiki website and scroll down to the "Download" area. You will find the latest stable release at the top of this section. Download the software and unzip it using any program with that functionality.
What other options do I have?
Offline Wikipedia readers – programs that allow reading of Wikipedia without being connected to the Internet. Some allow editing of pages, but most do not.