The time it takes for 50% of an unstable isotope to decay to a stable form is called the

The time it takes for 50% of an unstable isotope to decay to a stable form is called the half-life. 
A substance with longer half-life will be considered as more stable than those with shorter half life. Half-life mostly used for radioactive decay. Knowing the half-life you can count how much time needed for a radioactive waste to be cleared.