Pronunciation: (haf'līf", häf'-),[key] —n., —pl.-livesPronunciation: (-līvz").[key] 1. Physics.the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
2. Pharm.the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
3. Informal.a brief period during which something flourishes before dying out. Also,half' life",half'life".Also called half'-life pe"riod.