half-life

noun4

1. •
   The time required for half the nuclei in a sample of an 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 amount of time it takes for an idea or a fashion to lose half of its influential power.

   • Most books of scholarship have surprisingly short intellectual half-lives during which they make a difference" — 1991