Radioactive decay (a.k.a nuclear decay or radioactivity) is a process by which the nucleus of an unstable atom loses its energy by emitting ionizing radiation. Any material that spontaneously emits this kind of radiation (including alpha and beta particles, gamma rays and conversion electrons) is considered radioactive.
It is a random process in the case of single atoms, in that it is impossible to predict when a particular atom may decay, according to quantum theory. However, the chance that a given atom will decay never changes, regardless of how long that atom has existed. For larger numbers of atoms however, the rate of decay and the half-lives of the nuclides can be calculated. There are no known limits for shortness or length of duration for these numbers.