Chronon

Chronons were particles which make up time, similar to the way fermions make up matter and anti-matter or bosons make 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 between the empty spaces between chronons. (DW: The Time Monster)

Practical applications
The 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 trapped it in a time loop. (PDA: The Quantum Archangel)

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

In an alternate timeline Rose Tyler and UNIT used mirrors to bounce chronon energy off of 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)