A link on a Webpage that points to another page that no longer exists on the World Wide Web. This could be for any reason ranging from the other site changing and not doing 301 redirects to the other site closing down entirely. The link may have even been incorrect since inception and never pointed to a valid page due to an error typing it onto the site.
Linking to resources that no longer exist can cause significant frustration for users and good authority sites take what measures they can to minimise the impact of this including some automated checks for broken links and manual checking of some important links in key articles. Link rot (the problem caused by dead links) is a significant issue facing published work and some studies have shown as many as 3% of published links can vanish within a year in some fields.