A wiki (pronounced wik-ee) is a website whose users can add, modify, or delete its content via a web browser using a simplified markup language or a rich-text editor. Wikis are typically powered by wiki software and are often created collaboratively by multiple users. Examples include community websites, corporate intranets, knowledge management systems, and note taking.
Wikis may serve many different purposes. Some permit control over different functions (levels of access). For example, editing rights may permit changing, adding or removing material. Others may permit access without enforcing access control. Other rules may also be imposed for organizing content.
Ward Cunningham, the developer of the first wiki software, WikiWikiWeb?, originally described it as “the simplest online database that could possibly work.” “Wiki” is a Hawaiian word meaning “fast” or “quick.”
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For a great video on wiki basics see, Wikis in Plain English.