Half-life, in general terms, is the time required for half of something to undergo a process. The term has several uses but in all of those uses it means essentially the same thing. In dealing with radioactive isotopes, the term refers to the time required for half of the atoms of a radioactive substance to disintegrate. It can also be used in conjunction with ecological systems such as drug or radioactive tracer dissipation in the human body. In these circumstances it is the time required for half of the drug or tracer to be removed from the body. Half-life is also commonly used in the study of chemical kinetics. Here it is defined to be the time required for the reactant concentration to decay to one-half its initial value.

The half-life of radioactive isotopes is as varied as the number of isotopes known. Astatine, the heaviest known halogen...