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A hyperlink, or simply a link, is a reference in a hypertext document to another document or other resource. As such it would be similar to a citation in literature. However, 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.

A link has two ends, called anchors, and a direction. The link starts at the source anchor and points to the destination anchor. However, the term link is often used for the source anchor, while the destination anchor is called the link target.

The most common link target is a URL used in the World Wide Web. This can refer to a document, e.g. a webpage, or other resource, or to a position in a webpage. The latter is achieved by means of a HTML element with a "name" or "id" attribute at that position of the HTML document. The URL of the position is the URL of the webpage with "#attribute name" appended.

A web browser usually displays a hyperlink in some distinguishing way, e.g. in a different colour, font or style. A mouse cursor 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. If the target is not a html-file, depending on the file type and on the browser and its plug-ins, an other program may be activated to open the file.

The HTML code contains the three main characteristics of a link:

It uses the HTML element "a with the attribute "href" and optionally also the attribute "title":

link label

When the cursor hovers over a link, depending on the browser, some informative text about the link is shown:

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.

While hyperlinking among pages of Internet content has long been considered an intrinsic feature of the Internet medium, some websites have claimed that linking to them is not allowed without permission, see e.g. [1] and [1] (in Dutch). You do not need to ask permission to link to any page of Wikipedia's.

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 [1], 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.

See Wikipedia:Hyperlink for the usage of "Hyperlink" in Wikipedia.