< /ˈhæfˌlaɪvz, ˈhɑf-/.

  1. Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
  2. Also called biological half-life. Pharmacology. 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.


  1. the time taken for half of the atoms in a radioactive material to undergo decaySymbol: τ
  2. the time required for half of a quantity of radioactive material absorbed by a living tissue or organism to be naturally eliminated (biological half-life) or removed by both elimination and decay (effective half-life)

also halflife, half life, 1864, with meaning “unsatisfactory way of living;” the sense in physics, “amount of time it takes half a given amount of radioactivity to decay” is first attested 1907.


  1. The time required for half the nuclei of a specific radionuclide or radioactive substance to undergo radioactive decay.physical half-life
  2. The time required for half the quantity of a drug or other substance deposited in a living organism to be metabolized or eliminated by normal biological processes.biological half-life
  3. The time required for the radioactivity of material taken in by or administered to an organism to be reduced to half its initial value by a combination of biological elimination processes and radioactive decay.effective half-life

  1. The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity. For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.

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.

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