Fission
The decay of a heavy nucleus into several smaller fragments under the effect of a collision with a neutron. The mass of the reaction products is inferior to the mass of the initial nucleus. Missing mass M is not lost, it converts into energy E (carried away by the reaction products) through Einstein's formula E=Mc2. The factor c2, a gigantic number, explains why fission nuclear reactions are significant sources of energy used in various applications, be they civil or military.