What does the half-life of a radioisotope refer to?

The radioisotope is radioactive and it decays with a constant decay rate. On decaying it gives off energy and matter. The rate of decay of radioactive isotope is measured in terms of its half-life. This is the time when it becomes half of its initial value. It is represented as t_{1/2}.

Since, all radioactive reactions follow first order kinetics the half life is related to rate constant of the reaction as follows:

K=/frac{0.6932}{t_{1/2}}