The warp core is the matter and antimatter reaction assembly at the heart of a starship, supplying the energy that drives the warp nacelles and most onboard systems.
A warp core brings matter, typically deuterium, together with antimatter inside a reaction chamber where dilithium crystals regulate the annihilation. The resulting plasma is channelled to the nacelles to produce the warp field and also feeds power to the rest of the ship. Engineers monitor containment closely, because a failure can lead to a catastrophic breach.
Crews treat ejection of the core as a last resort. Voyager lost its core this way when B'Elanna Torres was forced to jettison it during a crisis, and the Enterprise-E ejected and detonated its core to seal a subspace tear created by a Son'a weapon.
A breach aboard the Enterprise-D over Veridian III forced the crew to separate the saucer moments before the stardrive section exploded. Engines have also behaved in unexpected ways, as when an untested cold-start restart devised by Spock and Montgomery Scott hurled the original Enterprise three days back in time.