Time loop

A time loop, known technically as a chronic hysteresis is a loop in time, in which the individuals are always experiencing the same events in an endless loop.

The Third Doctor attempted to explain the concept to the Brigadier and Jo Grant as a "loop in time". (DW: The Claws of Axos).

While there is no known technological method of escaping a time loop, the Doctor and Romana were able to break a time loop by deliberately recreating the events of the loop out of phase with the loop itself. (DW: Meglos)

Presumably this method could work on any time loop, but, while achievable in a time loop of two people in a small room, would be nearly impossible to do in a time loop on a planetary scale.

Notable uses of time loops

• Several planets were placed into time loops by the Time Lords, including the War Planet, the Vardan homeworld, and the Fifth Planet of the Sol system. (DW: The War Games, NA: Timewyrm: Exodus, DW: The Invasion of Time, DW: Image of the Fendahl, EDA: The Taking of Planet 5)
• Axos was placed into a time loop. (DW: The Claws of Axos)
• While the completed Key to Time would have stopped time altogether, five pieces and a makeshift sixth made of chronodyne was enough to generate a temporary time loop. (DW: The Armageddon Factor)
• Meglos used a time loop to trap the Doctor and Romana in the TARDIS to prevent them reaching Tigella.
• Captain Jack Harkness was trapped in a time loop with his fellow renegade Time Agent, Captain John for five years. (TW: Kiss Kiss, Bang Bang)