half-life 
noun
1The time taken for the radioactivity of a specified isotope to fall to half its original value: iodine-131 has a half-life of 8.1 days...
- The small, lead-shielded canisters contain pellets or a fine powder of caesium 137, which emits powerful gamma radiation with a half-life of 30 years.
- Tritium is radioactive, with a half-life of 12.26 years.
- The FDA recommends keeping a fourteen-day supply on hand; radioactive iodine has a half-life of eight days.
1.1The time required for any specified property (e.g. the concentration of a substance in the body) to decrease by half.Frova may represent an important advancement in the treatment of migraine headache as no other currently marketed treatment has a half-life of more than six hours....
- Prealbumin, also known as transthyretin, has a half-life of only 1 to 2 days.
- At age 18, caffeine has a half-life of 4.5 hours, which increases with age.