Dead link

A dead link (also called a broken link or dangling link) is a link on the World Wide Web that points to a web page or server that is permanently unavailable. 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. The browser may also return a DNS error indicating that a web server could not be found at that domain name. A link might also be dead because of some form of blocking such as content filters or firewalls.

Another type of dead link is a URL that points to a site unrelated to the content sought. This can sometimes 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.

Link rot is the process by which links on a website gradually become irrelevant or broken over time as sites they link to disappear, change content, or redirect to new locations.

Links specially crafted to not resolve, as a type of meme, are known as Zangelding, which roughly translated from Dutch means tangle thing. A zangelding is basically a list of self referencing broken links.

Dead links commonplace on the Internet can also occur on the authoring side, when website content is assembled, copied, or deployed without properly verifying the targets, or simply not kept up to date. 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.
http://en.wikipedia.org/wiki/Dead_link