The spontanous decay of an unstable nucleus by the emission of ionising radiation. It is a random process so we cannot predict when it will occur.
The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This process is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation.
Over time, as the unstable atoms in a source of radiation change, the activity of the source goes down because there are fewer unstable atoms present to decay.
It is not possible to predict when an individual atom might decay, but you can measure how long it takes for half the atoms to decay. This is called the half-life of this type…