1. One of two or more forms of a chemical element having the same properties and the same atomic number but different mass numbers or atomic weights.
2. Isotope literally means, "the same place, an equal place".
3. Elements that occupy the same place in the periodic table of the elements, sharing common chemical properties, but having different atomic weights (depending on the number of neutrons carried by each nucleus).

The decay rates of a number of radioactive isotopes are valuable clocks for archeologists, paleontologists, and geologists.