half-life (t½) [AS, haelf + lif] , 1. the time required for a radioactive substance to lose 50% of its activity through decay. Each radionuclide has a unique half-life. Also called radioactive half-life. 2. the amount of time required to reduce a drug level to half of its initial value. Usually the term refers to time necessary to reduce the plasma value to half of its initial value. After five half-lives, 97% of a single drug dose will be eliminated. See also biological half-life, effective half-life.
