/ˈīsəˌtōp/
Noun, pl. isotopes
1. Variants of a particular chemical element: while all isotopes of a given element share the same number of protons and electrons, each isotope differs from the others in its number of neutrons. (wikipedia.org)
2. (Physics) 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 for the same isotope will have the same atomic number but a different mass number (atomic weight). (wiktionary.org)
3. (Radiobiology) One of several nuclides having the same number of protons in their nuclei and hence having the same atomic number, but differing in the number of neutrons and therefore, in the mass number. Almost identical chemical properties exist between isotopes of a particular element. The use of this term as a synonym for nuclide is to be discouraged. (biology-online.org)
4. One of two or more atoms with the same atomic number but with different numbers of neutrons. one of two or more atoms with the same atomic number but with different numbers of neutrons
Word origin: Introduced by British chemist Frederick Soddy (1877-1956) on suggestion of Margaret Todd, from Greek isos “equal to, the same as” + topos “place”