Wikipedia is a multilingual, Web-based, free content encyclopedia project. Wikipedia's articles provide links to guide the user to related pages with additional information. Wikipedia is written collaboratively by volunteers from all around the world. Anyone is welcome to add information, cross-references or citations. Since its creation in 2001, Wikipedia has grown rapidly into one of the largest reference Web sites.

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The Columbia Encyclopedia, Sixth edition, at Bartleby.com contains nearly 51,000 entries (marshalling six and one-half million words on a vast range of topics), and more than 80,000 hypertext cross-references.