Half-life

The time in which half the atoms of a given quantity of a particular radioactive substance disintegrate to another nuclear form. Measured half-lives vary from millionths of a second to billions of years. Similarly, the time in which half the molecules of a chemical substance disappear as a result of chemical or biochemical transformation.

Numbered list of references

http://www.atsdr.cdc.gov/glossary.html#G-D - 
http://rais.ornl.gov/home/glossary.html#C  
http://sra.org/resources_glossary_g-i.php