One of two or more atoms with the same atomic number but with different numbers of neutrons WordNet 3.0 & Open English WordNet

Isotopes are distinct nuclear species of the same chemical element. Wikipedia

Nuclides having the same atomic number but different mass numbers Wikidata

Any of two or more atoms with the same atomic number that contain different numbers of neutrons. OmegaWiki

Any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei. As a consequence, atoms of isotopes will have the same atomic number but a different mass number. Wiktionary

Atoms of the same element having a different number of neutrons. Wiktionary (translation)