Half-life (t½) is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period. While the term "half-life" can be used to describe any quantity which follows an exponential decay, it is most often used within the context of nuclear physics and nuclear chemistry—that is, the time required, probabilistically, for half of the unstable, radioactive atoms in a sample to undergo radioactive decay.
The original term, dating to Ernest Rutherford's discovery of the principle in 1907, was "half-life period", which was shortened to "half-life" in the early 1950s. Rutherford applied the principle of a radioactive elements' half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206.
Half-life is used to describe a quantity undergoing exponential decay, and is constant over the lifetime of the decaying quantity. It is a characteristic unit for the exponential decay equation. The term "half-life" may generically be used to refer to any period of time in which a quantity falls by half, even if the decay is not exponential. The table on the right shows the reduction of a quantity in terms of the number of half-lives elapsed. For a general introduction and description of exponential decay, see exponential decay. For a general introduction and description of non-exponential decay, see rate law. The converse of half-life is doubling time.