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Half-life

1. (Science: 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. (Science: 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.