Lifetime
An unstable body decays to form more stable entities. The exact decay time is random but the law of probability governing this phenomenon is well known. Its mathematical expression depends on a single parameter called lifetime. On average, the higher the lifetime, the longer the unstable body takes to decay. Lifetime can greatly vary from one body to another: a fraction of a second for the muon, some 15 minutes for an isolated neutron, over 4 billion years for the Uranium-238 nucleus.