Hii Rohan!
This is mainly concerned with the Nuclear concept. The mass defect refers to difference in mass between an atom and some of the masses of protons, electrons and neutrons of an atom. The missing mass is the energy released during formation of atomic nuclei in fusion or fusion processes. Hope this helps.
Good luck!

Hello!!

the amount by which the mass of an atomic nucleus differs from the sum of the masses of its constituent particles, being the mass equivalent of the energy released in the formation of the nucleus. Words nearby mass defect. ... Compare binding energy.

Thanks