Толковый словарь английского языка - half-life

noun Date: 1907 the time required for half of something to undergo a process: as, the time required for half of the atoms of a radioactive substance to become disintegrated, the time required for half the amount of a substance (as a drug, radioactive tracer, or pesticide) in or introduced into a living system or ecosystem to be eliminated or disintegrated by natural processes, a period of usefulness or popularity preceding decline or obsolescence