The trash exhaust was a compartment aboard a Constitution-class starship. Crew could access this compartment by opening a hatch and climbing down a ladder. The internal space of the exhaust was circular. An exhaust had expulsion capability, which could jettison trash free of a ship's gravity well. There were at least twenty of these exhausts, with trash exhaust 20 located in the engineering hull. An exhaust's activation sequence could be controlled and monitored by personnel on the bridge.
In 2259, on board the USS Enterprise, Captain James T. Kirk and Khan Noonien Singh were launched from trash exhaust 20. They were attempting to board the USS Vengeance and were relying on the expulsion capability of this exhaust to give them the boost needed to reduce travel time. (Star Trek Into Darkness)