Answer : It is defined as the amount of time taken by a radioactive material to decay to half of its original value.
Explanation :
Half life : It is defined as the amount of time taken by a radioactive material to decay to half of its original value.
All radioactive decays follow first order kinetics.
The relation between the half-life and rate constant is:

The half-life of the radioactive isotope is remains the same that means a constant value and it is not affected by any external conditions.