According to Dictionary.com, radioactive means of, pertaining to, exhibiting, or caused by radioactivity. The term derives from the French "radio-actif," which was coined by Pierre and Marie Curie.
According to Encyclopædia Britannica, radioactivity is a property of certain kinds of matter involving the emission of energy and subatomic particles spontaneously. Unstable atomic nuclei decompose in order to reach a more stable state, thereby producing radioactive particles or electromagnetic radiation.
Half-life is the unit measurement for radioactive decay, where the time it takes for one-half of an element or isotope to decompose is determined. Half-lives range from fractions of a second to over a billion years.