Atoms of the same element that have the same number of protons (same atomic
number) but different numbers of neutrons (different atomic masses).
Isotopes have the same chemical properties but have different physical and
nuclear properties.
Examples of isotopes are plutonium-238, plutonium-239, plutonium-240, and
plutonium-241. Each acts chemically like plutonium but they have 144, 145, 146,
and 147 neutrons, respectively.
Source: GreenFacts
More:
A radioisotope is a radioactive isotope, i.e. an unstable isotope that
undergoes radioactive decay toward a more stable form, emitting
radiation.
