1. The half-life of a radioactive substance is a characteristic constant that measures the time it takes for a given amount of the substance to become reduced by half as a consequence of decay, and therefore, the emission of the radiatiom
2. The biological half-life or terminal half-life of a substance is the time it takes for a substance (for example a metabolite, drug, signalling molecule, radioactive nuclide, or other substance) to lose half of its pharmacologic, physiologic, or radiologic activity. In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve (plasma half-life) its steady-state
1 http://atomic.lindahall.org/what-is-meant-by-half-life.html (1.); https://en.wikipedia.org/wiki/Biological_half-life (2.)