Semantic link
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A semantic link is a typed link where the element itself provides meaningful information about the link (semantics). For instance, in XML, you might have a BookTitle element <BookTitle isbn="0812511816"/>. You can use the semantic information provided in the element (the ISBN) to create a link from this element to an online bookseller. The link describes some external relationship or issue.
An example of a semantic link would be (any syntax expressing) "A is-mother-of B". Some other relations like "B is-child-of A" may be implied by it.
Such links are the basis of a semantic network.
The Semantic Web as described by Tim Berners-Lee is one example of the many attempts over many decades to define standards for semantics of links.
Among the earliest were those used by early hypertext systems in the 1980s such as KMS, NoteCards and IBIS.
HTML included a rel and rev attribute in its hyperlinks to store additional information about the nature of the links. There was at least one attempt (in 1994) to define a set of standard rel and rev attributes to attach to HTML anchor tags to make them into typed links that would express semantic properties of the link. Although this IETF RFC failed, it set the standard for many other attempts to follow. Over the next ten years, a great many attempts to set standards for particular subsets of the World Wide Web rose and fell.
[edit] Microformats
These rel and rev attributes of the HTML links are used by some Microformats to add meaning to relationships. XHTML Friends Network (XFN), for instance, uses the "rel" attribute to allow web authors to identify their relationship to people they link from their websites. Other Elemental Microformats use these too - Vote Links use rev rev="vote-for", rev="vote-against" or rev="vote-abstain"; Tags use rel="tag", Directory uses rel="directory".
