Energy equivalent to mass defect is called binding energy. A nucleus is like a rigid spherical ball formed by bringing together a large number of tiny spherical balls in the form of nucleons.
Something is needed to keep the nucleons together. That something is binding energy which serves the purpose of glue. To provide the binding energy each nucleon has to contribute some of its mass resulting in mass defect.
"Binding energy is a measure of the strength of the bondage. More binding energy means more strong binding."
Binding Energy Formula is expressed as
Binding energy is the energy equivalent of the mass deficiency, energy released in the formation of an atom from subatomic particles. Larger the mass defect, greater the binding energy and more stable is the nucleus.