A radioactive isotope. An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation. More that 1300 natural and artificial radioisotopes have been identified.
The radioactive form of an element that has all the chemical properties of the stable form of the element is a radioisotope. The radioisotope undergoes radioactive decay.
A radioactive form of an element. Used in certain diagnostic tests.