A backlink for a given web resource is a link from some other website (the referrer) to that web resource (the referent). A web resource may be (for example) a website, web page, or web directory.
A backlink is a reference comparable to a citation. The quantity, quality, and relevance of backlinks for a web page are among the factors that search engines like Google evaluate in order to estimate how important the page is. PageRank calculates the score for each web page based on how all the web pages are connected among themselves, and is one of the variables that Google Search uses to determine how high a web page should go in search results. This weighting of backlinks is analogous to citation analysis of books, scholarly papers, and academic journals. A Topical PageRank has been researched and implemented as well, which gives more weight to backlinks coming from the page of a same topic as a target page. 
Some other words for backlink are incoming link, inbound link, inlink, inward link, and citation.
Backlinks are offered in Wikis, but usually only within the bounds of the Wiki itself and enabled by the database backend. MediaWiki, specifically offers the "What links here" tool, some older Wikis, especially the first WikiWikiWeb, had the backlink functionality exposed in the page title.
Search engines often use the number of backlinks that a website has as one of the most important factors for determining that website's search engine ranking, popularity and importance. Google's description of its PageRank system, for instance, notes that "Google interprets a link from page A to page B as a vote, by page A, for page B." Knowledge of this form of search engine rankings has fueled a portion of the SEO industry commonly termed linkspam, where a company attempts to place as many inbound links as possible to their site regardless of the context of the originating site. The significance of search engine rankings is pretty high, and it is regarded as a crucial parameter in online business and the conversion rate of visitors to any website, particularly when it comes to online shopping. Blog commenting, guest blogging, article submission, press release distribution, social media engagements, and forum posting can be used to increase backlinks.
Websites often employ SEO techniques to increase the number of backlinks pointing to their website. Some methods are free for use by everyone whereas some methods, like linkbaiting, require quite a bit of planning and marketing to work. There are also paid techniques to increase the number of backlinks to a target site. For example, private blog networks can be used to purchase backlinks.
There are several factors that determine the value of a backlink. Backlinks from authoritative sites on a given topic are highly valuable. If both sites and pages have content geared toward the topic, the backlink is considered relevant and believed to have strong influence on the search engine rankings of the web page granted the backlink. A backlink represents a favorable 'editorial vote' for the receiving webpage from another granting webpage. Another important factor is the anchor text of the backlink. Anchor text is the descriptive labeling of the hyperlink as it appears on a web page. Search engine bots (i.e., spiders, crawlers, etc.) examine the anchor text to evaluate how relevant it is to the content on a webpage. Anchor text and webpage content congruency are highly weighted in search engine results page (SERP) rankings of a webpage with respect to any given keyword query by a search engine user.
Changes to the algorithms that produce search engine rankings can place a heightened focus on relevance to a particular topic. While some backlinks might be from sources containing highly valuable metrics, they could also be unrelated to the consumer's query or interest. An example of this would be a link from a popular shoe blog (with valuable metrics) to a site selling vintage pencil sharpeners. While the link appears valuable, it provides little to the consumer in terms of relevance.
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