Aside from being the amount of time it takes a radioactive sample to transmute, it should also be noted that in quantum physics there are special rules for half-lives of singular atomic samples. After the given half-life, there is a 50% chance that the atomic sample has decayed completely, but there is no way to tell until the sample is checked. Until then, it is assumed that the atom is superposed, or rather in between existence and decay. If one were to hook up a device witch detects radiation emissions to some cyanide and thow the whole mess in a box with a cat, one could get something very interesting.