Suppose that y=Aekt, where A (>0) and k are constants, and t represents some measurement of time (see exponential growth). When k <0, y can be said to be exhibiting exponential decay. In such circumstances, the length of time it takes for y to be reduced to half its value is the same, whatever the value. This length of time, called the half-life, is a useful measure of the rate of decay. It is applicable, for example, to the decay of radioactive isotopes.
http://www.edumedia-sciences.com/a100_l2-radioactive-decay.html An animation showing the physical decay and corresponding graph of three radioactive isotopes. (Subscription)