&quot;A half-life often describes the decay of discrete entities, such as radioactive atoms. In that case, it does not work to use the definition &quot;half-life is the time required for exactly half of the entities to decay&quot;. For example, if there is just one radioactive atom with a half-life of 1 second, there will not be &quot;half of an atom&quot; left after 1 second. There will be either zero atoms left or one atom left, depending on whether or not the atom happens to decay.<p>Instead, the half-life is defined in terms of probability. It is the time when the expected value of the number of entities that have decayed is equal to half the original number. For example, one can start with a single radioactive atom, wait its half-life, and measure whether or not it decays in that period of time. Perhaps it will and perhaps it will not. But if this experiment is repeated again and again, it will be seen that - on average - it decays within the half-life 50% of the time.&quot;