A moratorium is the suspension of a DVD or box set after it has been available for a certain amount of time. It was recently announced on Amazon.com that the first four Harry Potter DVDs are to be going moratorium soon, the first three films in December and the fourth in January.
Disney often put their films on moratorium, with re-releases on anniversaries of the films' releases.
Some film companies often re-release the DVDs in a box set, after a series of films have been completed.
When DVDs go on moratorium, they are often given a higher price, which doesn't usually go down well with customers, who feel that they are forced into paying more for a particular film because it will no longer be available. Some customers may not bother buying DVDs that are on moratorium, because they may suspect that film companies will ending up re-releasing the films anyway.