Nuclide

A nuclide is a class of atom: a collection of all atoms that have the same number of protons and neutrons in their nuclei, for example, 19F. Nuclear properties, such as neutron absorption cross-section, half-life of decay, nuclear spin, and atomic mass, are unique for each nuclide. Chemical properties depend only on the number of protons. A group of nuclides with the same number of protons is called an element.

"Nuclide" is probably what most people mean when they say "isotope," which is something different. Two nuclides are isotopes if they have the same number of protons, but a different number of neutrons. This is equivalent to stating that they belong to the same element, so it is possible to say, for example, "isotope of carbon." The big difference is that the word "isotope" expresses a relation between nuclides, so using it alone is not strictly correct: "decay of radioactive isotopes" should actually be "decay of radioactive nuclides."