The property of matter in which spontaneous emission of radiation from the nuclei of atoms occurs. The unstable nuclei of atoms, called radioisotopes, undergo radioactive decay in which the nuclei of atoms release some of their energy and become more stable. Typical radioactive decays are alpha, beta and gamma decay. Matter is considered radioactive when containing unstable isotopes. The level of its radioactivity depends on the proportion of unstable isotopes and their half-life. Because unstable isotopes are found almost everywhere, virtually everything in our environment is radioactive.