or half life, half·life
noun, plural half-lives [haf-lahyvz, hahf-]. /ˈhæfˌlaɪvz, ˈhɑf-/.
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British Dictionary definitions for half-life
Medical definitions for half-life
Scientific definitions for half-life
Cultural definitions for half-life
In physics, a fixed time required for half the radioactive nuclei in a substance to decay. Half-lives of radioactive substances can range from fractions of a second to billions of years, and they are always the same for a given nucleus, regardless of temperature or other conditions. If an object contains a pound of a radioactive substance with a half-life of fifty years, at the end of that time there will be half a pound of the radioactive substance left undecayed in the object. After another fifty years, a quarter-pound will be left undecayed, and so on.