Backlinks, also known as incoming links, inbound links, inlinks, and inward links, are incoming links to a website or web page. In basic link terminology, a backlink is any link received by a web node (web page, directory, website, or top level domain) from another web node.
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.
When HTML (Hyper Text Markup Language) was designed, there was no explicit mechanism in the design to keep track of backlinks in software, as this carried additional logistical and network overhead.
Most Content management systems include features to track backlinks, provided the external site linking in sends notification to the target site. Most wiki systems include the capability of determining what pages link internally to any given page, but do not track external links to any given page.
Most commercial search engines provide a mechanism to determine the number of backlinks they have recorded to a particular web page.
Other mechanisms have been developed to track backlinks between disparate webpages controlled by organizations that aren’t associated with each other. The most notable example of this is TrackBacks between blogs.