half life

   a unit of relative time measuring the rate at which a radioactive substance decays, or (more generally) the rate of decrease for any process that decreases exponentially. In the case of radioactivity, the half life is the time required for the activity to be reduced by half. After a second half life, the activity is again reduced by half, so it is then 1/4 the original activity. To reduce the activity to 0.1% of the original amount requires about 9.966 half lives.