Atoms with differing numbers of neutrons (and hence differing atomic weights) which are not radioactive, i.e. are not unstable. Molecules that are ‘labelled’ with relatively rare stable isotopes can be used as metabolic markers for nutrients as they enter the bloodstream and the body stores, and undergo turnover in the body. Measurement is by mass spectrometry.
An isotope that is nonradioactive. K40 is a stable isotope.