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A hyperlink, or simply a link, is a reference in a hypertext document to another document or other resource. It is similar to a citation in literature. Combined with a data network and suitable access protocol, it can be used to fetch the resource referenced. This can then be saved, viewed, or displayed as part of the referencing document.

The most common type of hyperlink is the URL used in the World Wide Web. A web browser usually displays a hyperlink in some distinguishing way, e.g. in a different colour, font or style. A mouse pointer may also change into a hand motif to indicate a link. In most browsers, links are displayed in underlined blue text when not cached, but underlined purple text when cached. When the user activates the link (e.g. by clicking on it with the mouse) the browser will display the target of the link.

The Google search engine uses PageRank, a measure of link popularity to determine which page should be ranked first. The more pages that have a hyperlink pointing to a page, the higher rank that page gets. It is actually slightly more complicated than that, see PageRank for more information.

British Telecom sued Prodigy under U.S. Patent No. 4,873,662 claiming that Prodigy infringed its patent on web hyperlinks. However, after costly litigation, a court found for Prodigy, ruling that British Telecom's patent did not actually cover web hyperlinks. [1] Hyperlinks were first described in 1945 in the landmark paper As We May Think, as well in the widely-known project Xanadu starting in the 1960s.

Some websites claim that linking to them is not allowed without permission, see e.g. [1] and [1] (in Dutch). See also deep linking.

Legal aspect

In some jurisdictions it is or was (for example the Netherlands, see Karin Spaink) held that hyperlinks are not merely references or citations, but are devices for copying web pages. Although this principle is generally rejected by digerati, the courts that adhere to it see the mere publication of a hyperlink that connects to illegal material to be an illegal act in itself, regardless of whether referencing illegal material is illegal.