A plasma vent was a component of warp drive technology allowing excess plasma to be ejected harmlessly into space.
The secondary plasma vent on Galaxy-class vessels had a triple redundant bypass meaning that the primary access junction was routed through the port transducer matrix. (TNG: "Thine Own Self")
In 2152, a cloaked Romulan mine attached itself to the hull of Enterprise NX-01 just behind the secondary plasma vent. (ENT: "Minefield")
Later the same year, a Retellian freighter expelled a cloud of dilithium hydroxyls and ionized pyrosulfates into Enterprise's path, clogging the plasma vents and preventing the ship from engaging its warp drive. (ENT: "Precious Cargo")
In late-2152, two Enolian patrol ships were disabled after Captain Jonathan Archer opened the plasma vents of the prison transport ship he was on and ignited the plasma cloud with weapons fire. (ENT: "Canamar")
In 2370, the lack of emissions from DaiMon Prak's D'Kora-class vessel's plasma vents indicated to Geordi La Forge its warp drive was completely inactive. (TNG: "Force of Nature")
On stardate 51932.4, while the USS Voyager was proceeding through a dense nebula cloud with the crew in stasis, Seven of Nine stated in her log entry that she would have to purge the auxiliary plasma vents in order for the ship to continue to operate effectively. (VOY: "One")
In 2374, Lieutenant Commander Jadzia Dax and Chief Miles O'Brien piloted the USS Rubicon through a plasma vent in order to gain access to the USS Defiant after the runabout had been shrunk in size due to a subspace compression anomaly. As a result, they learned the ship had been taken over by hostile Jem'Hadar and were subsequently able to action a plan to retake it. (DS9: "One Little Ship")