Half--life
The time in which one half of the atoms of a particular radioactive element disintegrate measured in time units (seconds, days, years millenniums)
Glossary Source
JRC EU CBRNe Glossary
(
EU Glossary on Chemical, Biological, Radiological, Nuclear, and Explosive risks
)
https://publications.jrc.ec.europa.eu/repository/bitstream/JRC128863/JRC128863_01.pdf