The half-life of an element (chemical, physical or biological) is the time when half of the element is degraded (by reaction or nuclear disintegration).
It should not be confused with half of the total degradation time.
When the half-life time of an element is emitted, the kinetics of the reaction is exponential.
The half-life time is independent of the initial quantity of the element under study.
Thus, after 2 half-life times, only 25 % of the element studied remains, after 3, 12.5 % etc., and after 3, 12.5 % etc., the element remains.
This half-life time is used for carbon-14 dating.
Carbon 14 is an isotope of carbon that is naturally found in the CO2 breathed by all living things. When a living being stops breathing, the amount of carbon 14 present in its body decreases.
By comparing the quantity of carbon 14 in a living being today with the quantity of carbon 14 in a dead object, we can find the age of death of the person.
If the body to be dated contains exactly 50% of the quantity of Carbon 14 compared to the reference living being. Then we can estimate that the object to be dated is 5,730 years old.