Water contains lots of hydrogen atoms, with mass of 1 atomic mass unit. (whatisnuclear.com)
So, now instead of carbon listed as being 12.0107 atomic mass units with a measurement uncertainty of about 0.0008, it has an official atomic weight of [12.0096; 12.0116], where the brackets and semicolon indicate an interval of atomic weights. (scientificamerican.com)
«Heavy water» is just that: water with a molecular weight of 20 rather than the normal 18 atomic mass units, or amu. (scientificamerican.com)