(1)   The time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)


  1. The time required for half of the nuclei in a sample of a specific isotope to undergo radioactive decay.
  2. In a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
  3. The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
  4. The time it takes for an idea or a fashion to lose half of its influential power.
    • 1991, Robert Ackerman, Introduction to Jane Ellen Harrison's Prolegomena to the Study of Greek Religion (1903)
      Most books of scholarship have surprisingly short intellectual half-lives during which they make a difference"