Incoming link

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 and sources of backlinks for a web page are among the factors that Google's PageRank algorithm evaluates in order to estimate how important the page is. The PageRank score is, in turn, 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.

Some other words for backlink are incoming link, inbound link, inlink, inward link, and sitation.

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.

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Wikipedia