A subatomic particle that normally exists only in subspace, whose presence in ordinary space usually signals an unnatural intrusion, as with the Caretaker's scanning beam in Voyager's pilot.
Tetryons are particles that belong to subspace and are unstable in normal space. Finding them is therefore a clue that something, natural or artificial, has broken through from another domain or that exotic technology is at work.
They are first described in The Next Generation episode Schisms, where tetryon emissions in a cargo bay lead the crew to a pocket of subspace from which aliens have been abducting them. In Voyager's opening story Caretaker, a coherent tetryon beam scans the ship just before the Caretaker's displacement wave hurls it into the Delta Quadrant.
Tetryon readings recur throughout Deep Space Nine and Voyager as evidence of cloaked ships, strange weapons and subspace disturbances.