What is the meaning of Half-life?

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

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

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

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

Source: wiktionary.org