Definitions by ChemMan

Isotope 

Isotopes are atoms of the same element which have differing number of neutrons.

This means they have the same number of protons.

Deuterium is an isotope of hydrogen.
Deuterium is hydrogen-2, i.e. hydrogen with a mass number of 2 (1 proton + 1 neutron).

Tritium is a rarer isotope of hydrogen, and is also known as hydrogen-3 (it has 1 proton + 2 neutrons).

Isotope by ChemMan May 12, 2010