Link Rot

From the Wikipedia:Link Rot

The 404 "not found" response is familiar to even the occasional Web user. When you click on a link, and get "404 error", you are probably experiencing "link rot".

Link rot is the process by which links on a website gradually become irrelevant or broken as time goes on, because websites that they link to disappear, change their content or redirect to new locations. The phrase also describes the effects of failing to update web pages so that they become out-of-date, containing information that is old and useless, and that clutters up search engine results. This process most frequently occurs in personal web pages and is prevalent in free web because there is no financial incentive to fix link rot.