Antitachyon

From Memory Alpha, the free Star Trek reference.

An antitachyon is an elementary anti-particle often associated with fluctuations in the spacetime continuum. Unlike its counter, it appears that it does not only exist at faster-than-light velocities.

A pulse was emitted from the USS Voyager's deflector by Captain Janeway to close a temporal rift created by her Admiral alternate self from the future. (VOY: "Endgame")