The tetryon compositor is part of a Romulan-designed cloaking device. Damage to the compositor can prevent the cloak from working.

On orders from Vice Admiral Toddman, Michael Eddington sabotaged the tetryon compositor on the USS Defiant's cloaking device, to try and force Benjamin Sisko to abandon his insubordinate mission to rescue Odo. The cloak was disabled, but Sisko resolved the difficulty and was successful in reactivating the cloak, and completing the mission. (DS9: "The Die is Cast")

Apparently, this device has something to do with the subspace-based subatomic particles, tetryons.