Half-life

Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
Also called biological half-life. Pharmacology. the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
Informal. a brief period during which something flourishes before dying out.
Contemporary Examples

Far From the Streets, the Bold-Faced Names Rub Shoulders With the Wall Street Occupiers Jeff Smith March 25, 2012
ecoATM offers consumers a new way to sell used cell phones and electronic devices Sarah Langs August 31, 2013
The BlackBerry’s Death Rattle Gets Louder Daniel Gross August 11, 2013

Historical Examples

The Atomic Fingerprint Bernard Keisch
Woman in Modern Society Earl Barnes
Fantasia of the Unconscious D. H. Lawrence
Zero Data Charles Saphro
Life’s Basis and Life’s Ideal Rudolf Eucken
Now We Are Three Joe L. Hensley
They Also Serve Donald E. Westlake

noun
the time taken for half of the atoms in a radioactive material to undergo decay τ
the time required for half of a quantity of radioactive material absorbed by a living tissue or organism to be naturally eliminated (biological half-life) or removed by both elimination and decay (effective half-life)
n.

half-life
(hāf’līf’)
The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity. For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.

Note: Scientists can estimate the age of an object, such as a rock, by carefully measuring the amounts of decayed and undecayed nuclei in the object. Comparing that to the half-life of the nuclei tells when they started to decay and, therefore, how old the object is. (See radioactive dating.)