The antimatter purge vent was an integral component of the warp propulsion system of Federation starships. During an emergency situation, the antimatter could be removed from the ship through this vent.
Aboard the Galaxy-class USS Enterprise-D, in the Engineering Systems Database, there was a folder titled Antimatter Purge Vent Procedures. This folder contained technical manuals relevant to the topic. (TNG: "Booby Trap", okudagram)