The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in the study of radioactive decay which is subject to exponential decay but applies to all phenomena including those which are described by non-exponential decays. The term half-life was coined in 1907, but it was always referred to as half-life period. It wasn't until the early 1950s that the word period was dropped from the name.… (More on Half-life)