A catastrophic event where an airplane collides with an object, the ground, or the ocean. In contrast to a car crash, more likely than not all persons on board are dead due to the extraordinary-high magnitude of impact.
Such examples included the break-up of TWA Flight 800, the disappearance of Air France Flight 447 and ValuJet Flight 592.
It's hard to imagine that airplanes are actually the safest form of travel. The thing is, in a plane crash - everyone dies. At least in a car crash there are different levels of injury, possibly but not likely death.