Space-time portal

A space-time portal was a portal leading from one point in space and time to another.

History
When he was at the Time Lord Academy, the Doctor used to "muck about with" space-time portals. (PROSE: Made of Steel)

In 2008, Cybermen survivors of the Battle of Canary Wharf (TV: Doomsday) attempted to use Martha Jones as a hostage in order to force the Tenth Doctor to reopen the Void and release the Cyber-Army trapped there. Hooking up the Cybermen's teleportation equipment to the TARDIS, the Doctor apparently complied, but instead opened a space-time portal to the Jurassic era. A Tyrannosaurus rex emerged through the portal and destroyed two of the three Cybermen, dragging one of the three back with it to the Jurassic. The Doctor subsequently used the power cable for the portal to electrocute and kill the last Cyberman.

In the aftermath, the Doctor explained how he had only pretended to open a gateway into the Void but instead created a space-time portal. Having previously visited the Jurassic era with Martha on a short field trip, the Doctor had set the portal to open at the exact moment and location that the TARDIS had dematerialized so that the lunging T-rex that had been after them would instead get the Cybermen. (PROSE: Made of Steel)