[half-life] n (1907) 1: the time required for half of something to undergo a process: as a: the time required for half of the atoms of a radioactive substance to become disintegrated b: the time required for half the amount of a substance (as a drug or radioactive tracer) in or introduced into a living system or ecosystem to be eliminated or disintegrated by natural processes
2: a period of usefulness or popularity preceding decline or obsolescence "slang usually has a short ~"