A fractionation factor is a ratio of ratios: the ratio heavy versus light isotopes in one compound with a similar ratio of another compound, e.g., when one compound is produced from another through a chemical reaction. For example, if a chemical fraction has water as an input and methane as an output, and a certain percentage of the water has a deuterium atom and a certain percentage of the resulting methane has a deuterium atom, the ratio of the two percentages is a fractionation factor.
αA-B = RA/RB
Other processes might result in such fractionation (called isotope fractionation or isotopic fractionation), such as phase changes. Thus if water freezes, heavy water may freeze at slightly different rate as non-heavy water and the ratio of heavy to normal may differ.