Any molecular entity in which the isotopic ratio of nuclides for at least one element deviates measurably from that occurring in nature. The term includes both isotopically substituted compounds (in which essentially all the molecules of the compound have only the indicated nuclide(s) at each designated position) and isotopically labeled compounds (a formal mixture of an isotopically unmodified compound with one or more analogous isotopically substituted compound(s).