The mass defect ($\Delta m$) is the difference between the total mass of the individual nucleons (protons and neutrons) and the actual mass of the stable nucleus. The mass of the nucleus is always slightly less than the sum of the masses of its constituents because some mass is converted into binding energy ($E=\Delta m{c}^2$) during the formation of the nucleus. This energy is released and accounts for the 'lost' mass.