A hyperlink on another website that points to your website. Backlinks are used to determine the ranking of a website, because the more links pointing to it (the more backlinks), the more popular. Google uses backlinks to compute its results page ranking (PageRank) by going several levels back. For example, a site with 1,000 backlinks is popular, but if thousands of sites link to all the sites with the backlinks, then the backlinks to the target site have even more value. See link popularity checker and Google.
(Internet) To link in this fashion.