Deuterium is an isotope of hydrogen, with the nucleus composed of a proton and a nucleus. It has a different neutron absorption spectrum from that of ordinary hydrogen. Deuterium is stable, although it is roughly 6000 times less abundant than common hydrogen (whose nucleus contains no neutrons).
Deuterated molecules (in which hydrogens have been partially or fully replaced by deuterium) have similar properties to their hydrogen monologues. For example, deuterated water has a boiling point of 101.4 degrees Celsius compared to normal water, with a boiling point of 100 Celsius.