Nuclides having the same atomic number but different mass numbers.
Sources:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2520 (https://doi.org/10.1351/pac199466122513)
Red Book, 3rd ed., p. 35 (https://doi.org/10.1515/pac-2014-0718)
See also:
Orange Book, 2nd ed., p. 230 (https://media.iupac.org/publications/analytical_compendium/)