Mandyville is where television characters go when they're no longer required but when the writers can't be bothered (or just forget) to write them out of the show.
The term originated on The West Wing discussion boards on the internet, though it's slowly seeping into general usage. One of the regular cast from the show's first season, Mandy Hampton (played by Moira Kelly with just the right amount of hysteria) completely disappeared sometime before the start of season two. No reference is ever made to her again over the show's next six seasons, even in the last three episodes of the series where every major character whose time had come and gone arrives for a curtain call. Mandy Hamption disappeared off the face of the planet and apparently out of the cast's characters' memories, as there were plot lines involving her that were left to deteriorate.
Fans of the show started calling the lonely place where characters disappear to Mandyville in her honor. I like to think of it as a one-horse town straight out of a spaghetti western where all these characters mill around and wait to be ressurected by slightly crazed fan fiction auteurs.