Antimatter containment is the process whereby antimatter and matter are kept separated by the use of magnetic fields to prevent the oppositely charged particles from colliding, which would result in the destruction of the matter, along with a violent and explosive release of energy.
Antimatter containment is a key function of a starship warp core which uses both antimatter and matter, released in controlled and balanced amounts, to provide energy which powers the ship's warp drive.
Galaxy class starships in the 24th century had a system of seven independent computer safetylocks to prevent antimatter containment from being breached, assuming there was no damage to the warp core components.