Isotopologue
Isotopologues are molecules that differ only in their isotopic composition. Simply, the isotopologue of a chemical species has at least one atom with a different number of neutrons than the parent.
An example is water, where some of its hydrogen-related isotopologues are: "light water" (HOH or H2O), "semi-heavy water" with the deuterium isotope in equal proportion to protium (HDO or HHO), "heavy water" with two deuterium isotopes of hydrogen per molecule (D2O or H2O), and "super-heavy water" or tritiated water (T2O or H2O), where the hydrogen atoms are replaced with tritium isotopes. Oxygen-related isotopologues of water include the commonly available form of heavy-oxygen water (H2O) and the more difficult to separate version with the O isotope. Both elements may be replaced by isotopes, for example in the doubly labeled water isotopologue D2O.