The fifth dimension, also called fifth-dimensional space, was another realm where there was no past or future but only the present, where one existed everywhere. Fifth-dimensional beings existed across all of time, and other beings that entered the fifth dimension would shortly die of dehydration. (COMIC: Tesseract) Access to the fifth dimension was equated with access to time travel, (AUDIO: Mission to Magnus) since it and the fourth dimension were the two dimensions of time in the universe. (PROSE: The Quantum Archangel) Susan Foreman considered space to be the fifth dimension in her equations. (TV: An Unearthly Child)
When the Doctor's TARDIS collided with an Acari ship, it split the fifth dimension in two, causing chaos and allowing Emily Winter to meet the Tef'Aree. Before she could return the TARDIS was destroyed in an event level chronal distortion, and the huon radiation was too strong to transport her to another planet. They returned her to the TARDIS a few seconds before it was destroyed, enabling her to prevent it. (COMIC: Tesseract)
When Es'Cartrss of the Tactire was expelled from the TARDIS matrix as it exploded, (TV: The Pandorica Opens) it was expelled into E-Space, adrift as stardust in a fifth-dimensional vortex. It made contact with the T'keyn, whose mothership existed in fifth dimensional E-Space and who later instructed their ships to return to the fifth dimension. (COMIC: Dead Man's Hand)
Whether all of E-Space is fifth dimensional is unclear.