A subatomic particle that naturally travels faster than light, used throughout Starfleet science for detecting cloaked ships and associated again and again with time travel and temporal disturbances.
Tachyons are particles that exist only at speeds above that of light. Because they interact with cloaking fields and with distortions in time, they turn up constantly in sensor work: a web of tachyon beams strung between ships exposed cloaked Romulan vessels in the Next Generation episode Redemption II.
In the Deep Space Nine episode Explorers, Benjamin and Jake Sisko sail a replica of an ancient Bajoran lightship that is caught by tachyon eddies in the Denorios Belt and flung to warp speed, supporting the legend that Bajorans reached Cardassia centuries earlier.
In the Voyager finale Endgame, an elderly Admiral Janeway uses a tachyon pulse from a stolen chrono-deflector to open a rift into the past.