Mass defect in nuclear physics refers to the phenomenon where the mass of a nucleus is less than the sum of its individual protons and neutrons. This discrepancy arises because energy is required to bind these particles together, known as binding energy, which correlates to a loss of mass according to Einstein's equation E=mc². The discussion clarifies that this does not violate the law of conservation of mass, as it applies to chemical reactions rather than nuclear processes. Additionally, it is noted that particles, including quarks, do not have a constant mass; their mass can vary based on their state, particularly when bound in a nucleus. Understanding mass defect is crucial for comprehending nuclear stability and energy release in nuclear reactions.