the ratio of the mass defect of an isotope to its mass number, equal to (M − A) ÷ A, where M is the atomic mass and A is the mass number: usually expressed as parts per 10,000
See packing fraction in American Heritage Dictionary 4
noun
The quotient of the algebraic difference between the isotopic mass and the mass number of a nuclide, divided by its mass number, often interpreted as a measure of stability.