In the Babylon 5 universe, a dimension of space beyond both normal space and hyperspace. The Vorlons developed a gateway to Thirdspace a million years ago, but it opened the door to a frightening race of aliens, and was eventually abandoned in hyperspace. It was rediscovered by Commander Ivanova and her squadron of Starfuries in 2261, towed back to B5, activated semi-accidentally, and ultimately destroyed.