From AMS Glossary
The time over which the number of radioactive nuclei of a given species decays exponentially to half its initial value.
Half-lives range from small fractions of a second to billions of years. Half-life is also applied to any exponential decay. That is, if a physical quantity N at any time t is related to its initial value N0 by
the half-life for this process is τ ln 2, where τ is the mean life.