Definition of "half-life"

(physics) The time required for half the nuclei in a sample of an isotope to undergo radioactive decay.  examples

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

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

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