Backlinks, also known as incoming links, inbound links, in links, and inward links, are incoming links to a website or web page. In basic link terminology, a backlink is any link received by one web page, directory, or top level domain from another web page.
Inbound links were originally important (prior to the emergence of search engines) as a primary means of web navigation; today, their significance lies in search engine optimization (SEO).
The number of backlinks is one indication of the popularity or importance of that website or page (for example, this is used by Google to determine the PageRank of a webpage).
Outside of SEO, the backlinks of a webpage may be of significant personal, cultural or semantic interest: they indicate who is paying attention to that page.
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 their 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 link spam, where a company attempts to place as many inbound links as possible to their site regardless of the context of the originating site.