Chronon

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

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

Practical applications
The Tenth Doctor called the Daleks' temporal prison by the alternative name of a "chronon loop" (DW: 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. (PDA: 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. (VG: City of the Daleks)

In an alternate 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. (DW: Turn Left)

Behind the scenes

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