An unstable isotope is a type of atom that has an unpredictable rate of decay. It can also be referred to as a radioactive isotope, or a radioisotope. These terms are often used interchangeably, as they all describe the same phenomenon. Another synonym for unstable isotope is a radionuclide. Radionuclides emit radiation as they decay, making them useful in numerous applications, including medicine, energy production, and environmental studies. Despite the numerous synonyms for unstable isotope, they all share one common characteristic: they are inherently unstable and will eventually decay to form a stable nucleus.