Atoms of the same element (i.e. having the same atomic number) which differ in mass number are called isotopes of that element. The isotopes of an element are identical in chemical properties, and in all physical properties except those determined by the mass of the atom. The different isotopes of an element contain different numbers of neutrons in their nuclei. Nearly all elements found in nature are mixtures of several isotopes.

(Uvarov, E. B., d. R. Chapman, and A. Isaacs. A Dictionary of Science. Third Edition. Penguin Books Ltd., Harmondsworth, England, 1966.)

The term "isotope" was found in the following pages:

Link database: Chemical Fact Sheet: Uranium
Toxic Mercury in Aquatic Life Could Spike due to Climate Change | EVISA's News
2006: Winter Conference on Plasma Spectrochemistry | EVISA's Agenda of Events
Material database: American Elements - Neodymium 145 Oxide Isotope
Material database: American Elements - Ruthenium 101 Metal Isotope
Material database: ISC Science - IES-Pb207 207Pb-enriched standard solution
PLSG Invests $100,000 in Applied Isotope Technologies, Inc., for Development of New Technologies for Environmental and Biological Analyses | EVISA's News
Material database: American Elements - Palladium 108 Metal Isotope
Material database: American Elements - Magnesium 25 Oxide Isotope
Speciation Analysis - Striving for Quality | EVISA's News
ICMGP 2015: 12th International Conference on Mercury as a Global Pollutant | EVISA's Agenda of Events
Link database: GEOROC Database (Geochemistry of Rocks of the Oceans and Continents)
Material database: American Elements - Silicon 28 Dioxide Isotope
Chromium speciation in solid matrices | EVISA's News
ICP-MS: A versatile detection system for trace element and speciation analysis | EVISA's News
Material database: American Elements - Lead 206 Carbonate Isotope
Instrument database: New Wave Research, Inc. - UP266 UV YAG Laser Ablation System
Company database: Applied Isotope Technologies
Instrument database: CPI International - Desolvation system APEX
Instrument database: Bruker Daltonics - apex-Qe - Qh-FTMS