Time loop

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

An accurate time loop took little energy to perpetuate. An incomplete time loop (where each repetition was not the same as the last or the loops were not the same length) required more energy to work, with the energy increasing with each loop. (NSA: The Pirate 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 was 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 Lords' 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. The Doctor placed the Marshal of Atrios in a time loop to prevent him from destroying Zeos. (DW: The Armageddon Factor)
 * Meglos used a time loop to trap the Doctor and Romana in th