half-life

Short definition from Wiktionary

Noun

1. 1
   The time required for half the nuclei in a sample of an isotope to undergo radioactive decay.
2. 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. 3
   The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
4. 4
   The amount of time it takes for an idea or a fashion to lose half of its influential power.