What is mass defect?

1. the difference between the mass of an isotope and its mass number, representing binding energy.
2. The observed atomic mass is slightly less than the sum of the masses of the protons, neutrons, and electrons that make up the atom. The difference, called the mass defect, is accounted for during the combination of these particles by conversion into binding energy, according to an equation in which the energy (E) released equals the product of the mass (m) consumed and the square of the velocity of light in vacuum (c); thus, E = mc2
3. Both definition are correct choose any of these
4. Hope I'm making myself clear do well in the future cheers!!