half-life

UK: /ˈhɑːflaɪf/ US: (haf′līf′, häf′-)

Inflections of 'half-life' (n): npl: half-lives

WordReference Random House Learner's Dictionary of American English © 2026

ˈhalf-ˌlife or ˈhalf ˌlife, n. [countable] , pl. -lives. 

1. Physics the time required for one half the atoms of a given amount of a radioactive substance to decay:
   a half-life of over 50,000 years.

WordReference Random House Unabridged Dictionary of American English © 2026

half-life  (haf′līf′, häf′-), n., pl. -lives 

(-līvz′). 
1. Physics the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
2. [Pharm.] the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
3. Informal Terms a brief period during which something flourishes before dying out.

Also, half′ life′, half′life′. Also called half′-life pe′riod. 

• half + life 1905–10

Collins Concise English Dictionary © HarperCollins Publishers::

half-life noun

1. the time taken for half of the atoms in a radioactive material to undergo decay