Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms per box (left) or 400 (right).

The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.

Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation. Rutherford applied the principle of a radioactive element's half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206. The original term, half-life period, dating to Ernest Rutherford's discovery of the principle in 1907, was shortened to half-life in the early 1950s. The converse of half-life is doubling time. For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body. The term is also used more generally to characterize any type of exponential or non-exponential decay.

The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable atoms survive. Half-life (symbol t 1⁄2) is the time required for a quantity to reduce to half of its initial value.