Chronon

Chronons were discrete particles of time (TV: Time and the Rani), particles which made up time, similar to the way fermions made up matter and anti-matter or bosons made up force.

Theory
At infinitesimal intervals, time actually existed in these discrete particles. TOMTIT transported matter by slipping it into the empty spaces between chronons. (TV: The Time Monster)

Practical applications
Chronons were released by time travel, but time travel also needed chronons to work. (AUDIO: The Time of the Daleks)

The Rani meant to create a planetary shell of chronons through the fusion of Helium 2 and the atmosphere of the planet Lakertya; this was the basis for the creation of a Time Manipulator. (TV: Time and the Rani)

The Tenth Doctor called the Daleks' temporal prison by the alternative name of a "chronon loop". (TV: Journey's End)

The Time Lords trapped the Mad Mind of Bophemeral in a single chronon following the Millennium War, in effect trapping it in a time loop. (PROSE: The Quantum Archangel)

Chronons could be blocked, either by a TARDIS or by a special-purpose device. Doing so could prevent a person or object from being erased from the present, even if their past no longer existed. (GAME: City of the Daleks)

In an alternative timeline, Rose Tyler and UNIT used mirrors to bounce chronon energy off Donna Noble, allowing her a one-way time jump to correct the change made to her time stream. Rose also implied the concentration of chronons used was directly relevant to how far the jump went. (TV: Turn Left)

Negative effects
Chronon particles were deleterious to the health of time travellers. After a Dahensa ranger crash-landed near Zarechny, Siberia in 1903 and began to emit chronon particles, the First Doctor and his granddaughter Susan Foreman both exhibited signs of radiation poisoning. (AUDIO: The Wanderer)

While the Fifth Doctor was travelling through the Time Vortex, the TARDIS detected dangerous levels of chronon particles, which were causing temporal distortions in local space, emanating for the Jariden Vault of Stellar Curios. He materialised his TARDIS in the Vault and offered his assistance to the Jaridens in order to correct the problem.

Considerably later in the Doctor's personal timeline, his eighth incarnation once again detected the emission of the particles from the Vault. Although his intended destination was the Petrachi Connection, his TARDIS nevertheless materialised in the facility as well. He remembered his younger self's visit, though his memories were initially rather vague. (AUDIO: The Four Doctors)

Behind the scenes

 * Chronons are used as a in-game currency in the MMORPG, Worlds in Time. They must be purchased with real money.