The Defence Indefinite Timeloop Option (or DITO) was a part of the safety systems of the Doctor's TARDIS from an alternate universe. It could be activated automatically by the TARDIS if it detected a threat, and caused the ship to project itself forwards a millisecond in time and remain there, inaccessible to intruders.
It was engaged when the Seventh Doctor landed in Oxford and the TARDIS detected the presence of the Garvond. The DITO could be deactivated using Bernice Summerfield's homing beacon. (PROSE: The Dimension Riders) The Doctor did not activate the option in the Land of Fiction, which alerted Ace that the TARDIS was missing. (PROSE: Conundrum)
The TARDIS, possibly, activated the DITO as it was in the process of exploding in order to save River Song, who was in the main control room at the time. The Eleventh Doctor was able to enter and leave the time loop with River's vortex manipulator. (TV: The Big Bang)