Heavy Hydrogen

Any atom that contains only one proton is hydrogen. This is, of course, the idea behind atomic number. Most hydrogen nuclei consist of only that one proton, however a very few contain either one or two neutrons.
A hydrogen atom with a single neutron has an atomic mass of approximately 2 amu's and is called deuterium.

A hydrogen with two neutrons has an atomic mass of approximately 3 amus and is called tritium.