The antiproton injection seal, or magnetic seals, were part of the matter/antimatter containment system aboard Federation starships in the mid-24th century. The seals prevented the antimatter from making contact with the ship's matter and destroying the vessel. These seals were located in both the antimatter chamber and the dilithium regulation chamber.
In 2365, the USS Yamato was destroyed following system failure when its seals failed. This was followed by the failure of the emergency system in ejecting antimatter from the ship in the event of a seal failure. (TNG: "Contagion")