half-life

1. The time required for half the nuclei in a sample of an isotope to undergo radioactive decay.

   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)

2. (physical)In a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.

   Halbwertszeit

   пери́од полужи́зниperíod polužízni

3. The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.

   пери́од полувыведе́нияperíod poluvyvedénija

4. The amount of time it takes for an idea or a fashion to lose half of its influential power.

regular