Tachyon

Tachyons were particles which could only travel faster than light, allowing them to travel in time. Tachyonics was the study of tachyons.

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

Tachyons could be used to allow a being to travel in time (PROSE: Trace Memory), to age or become younger. (TV: The Leisure Hive) They could affect the universe before the events that caused them occur, since they would travel to the past. (PROSE: Trace Memory)

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

The creation of solid tachyonic images was described by a set of wave equations. Practical realisation of these equations required the introduction of a random component in the tachyon emission, and the end result was a matching tachyon field that could create a temporary reduplication of any physical object. (TV: The Leisure Hive)

History
According to Romana II, the tachyon was first hypothesised on Earth, but, as the Fourth Doctor stated, "nobody there did much with it". The development of tachyonics to a useful state by 2290 was accomplished as a direct result of the war between the Argolins and the Foamasi forty years earlier. (TV: The Leisure Hive)

Uses
The Vondrax fed on tachyons. When their supply started to run out, they produced special orbs to store tachyons. (PROSE: Trace Memory)

The Argolins attempted to develop tachyon-based regeneration 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. (TV: The Leisure Hive)

Artificial tachyons in hyperspace, known as icarons, were developed during the Earth Empire. (PROSE: Original Sin)

TARDISes relied on several tachyon-based technologies, including the tachyon beam collimator. (PROSE: The Eye of the Giant) Tachyons could be detected by use of a tachyon meter. (AUDIO: The Death of Captain Jack)