The nuclear force (or nucleon–nucleon interaction or residual strong force) is the force between protons and neutrons, subatomic particles that are collectively called nucleons. The nuclear force is responsible for binding protons and neutrons into atomic nuclei. Neutrons and protons are affected by the nuclear force almost identically. Since protons have charge +1 e, they experience a strong electric field repulsion (following Coulomb's law) that tends to push them apart, but at short range the attractive nuclear force overcomes the repulsive electromagnetic force. The mass of a nucleus is less than the sum total of the individual masses of the protons and neutrons which form it. The difference in mass between bound and unbound nucleons is known as the mass defect.   Energy is released when some large nuclei break apart, and it is this energy that is used in nuclear power and nuclear weapons.

In particle physics, the strong interaction is the mechanism responsible for the strong nuclear force (also called the strong force, nuclear strong force), one of the four known fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. Despite only operating at a distance of a femtometer, it is the strongest force, being approximately 100 times stronger than electromagnetism, a million times stronger than weak interaction and 10³⁸ times stronger than gravitation at that range. It ensures the stability of ordinary matter, confining quarks into hadron particles, such as the proton and neutron, the largest components of the mass of ordinary matter. Furthermore, most of the mass-energy of a common proton or neutron is in the form of the strong force field energy; the individual quarks provide only about 1% of the mass-energy of a proton.