borderline between life and death; verge of death
Used for situations where someone is at the critical point between life and death, often in medical, accident, or combat contexts.
He hovered between life and death after the traffic accident.
Historical term for a boundary around a prison that inmates were forbidden to cross, often under penalty of being shot. Not used in modern contexts; the modern 'deadline' is 締め切り.
The doctors worked desperately to save the patient who had crossed the verge of death.
In old prisons, inmates who crossed the deadline could be shot dead.
Compound of 死 (death) and 線 (line). The prison sense is a calque from English 'deadline', originally referring to a physical line around a prison.