Answer:
HALF LIFE.
Step-by-step explanation:
The half life of a radioactive element refers to the length of time that is required for the half of the atoms of a particular sample of radioactive element to decay. For instance a radioactive element that a mass of 120 g and a half life of 2 years will be reduce to 60 g in two years. Half life is usually used to measure the rate of disintegration of an unstable isotope. The principle of half life of radioactive elements can be used to determine the relative ages of rocks.