Tachyon

Tachyons were a rare type of particle which could only travel faster than light, allowing it to travel in time.

Properties
Tachyons were produced whenever an parallel universe was produced. At the beginning of time, great amounts of tachyon radiation was created, since reality was still very fluid (the parallel universe could have greater gravity, slower light speed, etc.). As time went on, the potential changes to the universe became smaller and more negligible, and tachyons became rarer and rarer. (TWN: Trace Memory)

Tachyons could be used to allow a being to travel in time (TWN: Trace Memory) or to allow them to age or de-age (DW: The Leisure Hive). The Tachyons can have an affect before the events that cause them occur, since they would travel in time to the past. (TWN: Trace Memory)

Tachyons are the opposite to the radiations given off by time rifts. When the two radiations are combined, they violently repel each other. (TWN: Trace Memory)

Uses

 * The Vondrax fed on tachyons, but when their supply started to run out, they produced special orbs to store tachyons. (TWN: Trace Memory)
 * The Argolins attempted to develop tachyon-based regenerative process for the Leisure Hive. The Tachyon Recreation Generator was unsuccessful until it was helped by the Fourth Doctor. Pangol used tachyon technology to clone himself, bringing copies of himself from the future. Eventually, the process was reversed and he was turned into a child. (DW: The Leisure Hive)
 * Artificial tachyons in hyperspace, known as icarons, were developed during the Earth Empire. (NA: Original Sin)
 * TARDISes relied on several tachyon-based technologies, including the tachyon beam collimator. (MA: The Eye of the Giant)