A link that doesn’t work anymore is called a dead link, broken link or dangling link.
Link rot (or linkrot) is the process by which the collection of links on a website gradually point to web pages or servers that have become permanently unavailable. The phrase also describes the effects of failing to update out-of-date web pages that clutter search engine results.
Because broken links are, to some, very annoying, generally disruptive to the user experience, and can live on for many years, sites containing them are regarded as unprofessional.
A link may become dead for several reasons: The most common result of a dead link is a 404 error, which indicates that the web server responded, but the specific page could not be found. Some news sites contribute to the link rot problem by keeping only recent news articles online where they are freely accessible at their original URLs, then removing them or moving them to a paid subscription area. This causes a heavy loss of supporting links in sites discussing newsworthy events and using news sites as references.
Another type of dead link occurs when the server that hosts the target page stops working or relocates to a new domain name. In this case the browser returns a DNS error but may also display a site unrelated to the content sought. This can occur when a domain name is allowed to lapse, and is subsequently reregistered by another party. Domain names acquired in this manner are attractive to those who wish to take advantage of the stream of unsuspecting surfers that will inflate hit counters and PageRanking.