Empty dimension

An empty dimension was a "spare dimension" "every TARDIS" had as standard equipment. It was entirely featureless, and a conduit could be activated using a button in the TARDIS to imprison someone in the empty dimension for eternity, where they could not die. The Fourth Doctor tricked the Malevilus Magog into pressing the button in the Doctor's TARDIS, which activated the conduit, sucking him into the empty dimension. (COMIC: Doctor Who and the Iron Legion)