Definition - What does Radioactive mean?
A radioactive substance is one that emits a dangerous form of activity known as radioactivity. Radioactivity occurs when particles within atoms are emitted from the nuclei as a result of nuclear instability. This occurs in a process known as radioactive decay. There are different forms of radiation, depending upon the nuclear isotope, including alpha, bet and gamma.
Safeopedia explains Radioactive
In stable atoms, the force binding the protons and neutrons together is strong enough to result in a stable nucleus. In other atoms, such as uranium, however, this force is not strong enough, resulting in an unstable nucleus. The unstable nucleus emits particles in the process of radioactive decay. When enough particles have been emitted to create a new nucleus, radioactivity stops.