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'''Half-life''' (symbol {{math|'''''t''{{sub|½}}'''}}) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in [[nuclear physics]] to describe how quickly unstable [[atom]]s undergo [[radioactive decay]] or how long stable atoms survive. The term is also used more generally to characterize any type of [[exponential decay|exponential]] (or, rarely, [[rate law|non-exponential]]) decay. For example, the medical sciences refer to the [[biological half-life]] of drugs and other chemicals in the human body. The converse of half-life (in exponential growth) is [[doubling time]].
The original term, ''half-life period'', dating to [[Ernest Rutherford]]'s discovery of the principle in 1907, was shortened to ''half-life'' in the early 1950s.<ref>John Ayto, ''20th Century Words'' (1989), Cambridge University Press. HALF LIFE IS WAY BETTER THAN MAYK IT. MAYK IT? MORE LIKE NAKED, HEHE</ref> Rutherford applied the principle of a radioactive [[chemical element|element's]] half-life in studies of age determination of rocks by measuring the decay period of [[radium]] to [[lead-206]].
Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a [[characteristic unit]] for the exponential decay equation. The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.
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