The loss of antimatter containment in a starship's warp core, an explosion that destroys the vessel unless the core is shut down or thrown clear in time.
A warp core breach happens when the fields that contain a starship's matter and antimatter reaction give way. The blast that follows destroys the ship and anything close by, so a crew that cannot stop it has two choices: eject the core into space or get clear of it.
The Enterprise-D is lost this way in Star Trek Generations. Damage from the Duras sisters' Bird-of-Prey starts a coolant leak that cannot be stopped, the crew crowd into the saucer section and separate, and the breach destroys the engineering hull and throws the saucer down on to Veridian III.
Ejection is shown elsewhere. In the Voyager episode Day of Honor a failed experiment forces the crew to dump the core, and Paris and Torres are left adrift in spacesuits while trying to recover it. In Star Trek: Insurrection the Enterprise-E ejects its core and detonates it to seal a tear in subspace.