Glossary » link rot
because websites are frequently appearing and disappearing (and moving to new addresses), some fraction of links to outside websites will become invalid links over time. The slow degeneration of a web page with many links, due to the occasional link that becomes invalid, is known as “link rot”.
Avoiding link rot requires mechanisms of detecting dead links and updating them if they have moved. This maintenance can be extremely costly, encouraging many designers to be very cautious about creating links to websites not under their control and especially those that are not viewed as being stable, permanent addresses.