Wikipedia (pronounced /ˌwiːkiˈpiːdiə/ or /ˌwɪkiˈpiːdiə/) is a multilingual, Web-based, free content encyclopedia project. The name Wikipedia is a portmanteau (combination of words and their meanings) of the words wiki (a type of collaborative Web site) and encyclopedia. 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. Since its creation in 2001, Wikipedia has grown rapidly into one of the largest reference Web sites, attracting at least 684 million visitors yearly by 2008. There are more than 75,000 active contributors working on more than 10,000,000 articles in more than 250 languages. As of today, there are 2,507,053 articles in English; every day hundreds of thousands of visitors from around the world make tens of thousands of edits and create thousands of new articles to enhance the knowledge held by the Wikipedia encyclopedia. (See also: Wikipedia:Statistics).

Visitors do not need specialized qualifications to contribute, since their primary role is to write articles that cover existing knowledge; this means that people of all ages and cultural and social backgrounds can write Wikipedia articles. Most of the articles can be edited by anyone with access to the Internet, simply by clicking the edit this page link. Anyone is welcome to add information, cross-references or citations, as long as they do so within Wikipedia’s editing policies and to an appropriate standard. Substandard or disputed information is subject to removal. Users need not worry about accidentally damaging Wikipedia when adding or improving information, as other editors are always around to advise or correct obvious errors, and Wikipedia’s software is carefully designed to allow easy reversal of editorial mistakes.

Because Wikipedia is an ongoing work to which, in principle, anybody can contribute, it differs from a paper-based reference source in important ways. In particular, older articles tend to be more comprehensive and balanced, while newer articles more frequently contain significant misinformation, unencyclopedic content, or vandalism. Users need to be aware of this to obtain valid information and avoid misinformation that has been recently added and not yet removed (see Researching with Wikipedia for more details). However, unlike a paper reference source, Wikipedia is continually updated, with the creation or updating of articles on topical events within seconds, minutes or hours, rather than months or years for printed encyclopedias.