Radioactive decay and half life
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 describing how long it takes atoms to undergo radioactive decay but also applies in a wide variety of other situations.
Half-lives are very often used to describe quantities undergoing exponential decay—for example radioactive decay—where the half-life is constant over the whole life of the decay, and is a characteristic unit (a natural unit of scale) for the exponential decay equation.
The converse for exponential growth is the doubling time.