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A nuclide is beta-stable if it has no energetically allowed single beta-minus decay, beta-plus decay, or electron capture to a lower-mass neighboring nuclear isobar, with electron thresholds treated consistently. Beta stability need not imply stability against alpha decay, double beta decay, or nuclear fission.

Nuclear isobars are different nuclides with the same mass number $A$ and different atomic numbers $Z$. They differ from isotopes, which have fixed $Z$. Beta decay connects neighboring nuclear isobars.

A nuclear species specified by atomic number, mass number, and nuclear energy state. Isotopes compare nuclides with the same atomic number; changing only the nuclear energy state does not create a different isotope.