Half-life is a term used to describe the time required for a quantity to reduce to half of its initial value
. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable atoms survive. The half-life of a radioactive isotope is the amount of time it takes for one-half of the atomic nuclei of a radioactive sample to decay
. The half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation
. The term "half-life" is almost exclusively used for decay processes that are exponential or approximately exponential
. In biology and pharmacology, a biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radioactive activity
