1. <pharmacology> The period over which the concentration of a specified chemical or drug takes to fall to half its original concentration in the specified fluid or blood.

2. <radiobiology> The time required to reduce the amount of a radionuclide to one-half the amount originally present. Physical or radioactive half-life refers to reduction of activity by radioactive decay, biological half-life refers to biological elimination from the body and effective half-life refers to the combined action of radioactive decay and biological elimination.