Time loop

A time loop, known technically as a chronic hysteresis (TV: Meglos), was a loop in time in which the occupants experienced the same events in endless repetition.

Science
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) was unstable. Energy requirements increased with each iteration. (PROSE: The Pirate Loop)

While there was no known technological method of escaping a time loop, the Fourth Doctor and Romana broke a time loop by deliberately recreating the events of the loop out of phase with the loop itself. (TV: Meglos) The Third Doctor was able to escape the time loop he created around Axos by boosting the signal. This might indicate that the person creating a time loop could still leave it. (TV: The Claws of Axos) As he later related, however, due to the Time Lords tampering with his TARDIS and knowledge of time travel the loop he created was not an accurate time field/threshold (AUDIO: The Feast of Axos) so this may have allowed his exit.

Characteristics
The Third Doctor attempted to explain the concept to the Brigadier and Jo Grant as a "loop in time," the same events in space occurring over the same place in time. Time loops had the appearance of the infinity symbol, (TV: The Claws of Axos) a figure of eight looping through four dimensions.

The threshold of a time loop, possibly called the "time barrier," could be crossed into. Echoes of the past and future could be experienced, such as messages repeating themselves. This was evidence that various futures were possible; a message from the future did not guarantee those events would happen as time was in flux. A time loop would eventually decay naturally over six billion years. (AUDIO: The Feast of Axos)

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 Planet 5. (TV: The War Games, PROSE: Timewyrm: Exodus, TV: The Invasion of Time, TV: Image of the Fendahl, PROSE: The Taking of Planet 5)
 * Axos was placed into a time loop. (TV: 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 Fourth Doctor put the Marshal of Atrios's Command Capsule in a time loop to stop him from destroying Zeos. (TV: The Armageddon Factor)
 * Meglos used a time loop to trap the Fourth Doctor and Romana in the TARDIS and prevent them reaching Tigella. (TV: Meglos)
 * Jack Harkness was trapped in a time loop with his fellow renegade Time Agent John Hart for five years. (TV: Kiss Kiss, Bang Bang)
 * When the Brilliant was attacked by space pirates, the engine was engaged and accidentally caused an incomplete time loop. Though things were brought back after destruction, it was inconsistent and mistakes were made. The Tenth Doctor finally fixed the time loop, causing less damage. (PROSE: The Pirate Loop)
 * The Eighth Doctor and Charley Pollard got trapped inside a time loop and space loop in an Edwardian house. (AUDIO: The Chimes of Midnight)
 * The Trickster placed Sarah Jane Smith and her friends in a time loop. (TV: The Wedding of Sarah Jane Smith)
 * A crashed time ship trying to find a pilot for itself created several time loops during its failed attempts. (TV: The Lodger)
 * The Doctor's TARDIS put River Song in a time loop to stop its explosion killing her, until she was rescued by the Eleventh Doctor. (TV: The Big Bang)
 * In 2050, due to the Oroborus consuming time, time distortions such as time loops occurred across London. (TV: Oroborus)
 * The life cycle of the Cinder was a form of time loop; the creature was defeated by the First Doctor and Vicki Pallister in 1814. Its surviving form was accidentally taken back to the 12th century B.C. by Vicki. That form would eventually grow into the creature defeated in 1814. (AUDIO: Frostfire)
 * The Eighth Doctor set up a time loop inside Edward Fyne's body to temporarily slow down the progress of his tyger-fever. (PROSE: The Eye of the Tyger)