A half-life is the time it takes for exactly half of something to decay, transform, or disappear. The term is most commonly used in science to describe radioactive materials.

When scientists study radioactive substances like uranium or carbon-14, they measure how long it takes for half the atoms to break down into different elements. If a radioactive element has a half-life of 100 years, you'd start with 1,000 atoms, wait 100 years, and have 500 atoms remaining. Wait another 100 years and you'd have 250, then 125, and so on. The material keeps halving, never quite reaching zero but getting smaller and smaller.

Scientists use half-life to date ancient objects. Carbon-14 has a half-life of about 5,730 years, so by measuring how much remains in an old bone or piece of wood, researchers can calculate when a plant or animal died. Half-life also matters in medicine: doctors use radioactive tracers with short half-lives to see inside the body, choosing materials that disappear quickly so patients aren't exposed to radiation for long.

The term appears outside science too. People might say a joke has a short half-life if it stops being funny quickly, or that a phone battery has a half-life as it gradually loses its ability to hold a charge over years of use.