The principle that the number of baryons must remain the same in any nuclear reaction.
As an example consider the first reaction in the fusion process that the Sun uses to convert hydrogen into heliuum. This is a reaction that combines two protons into one deuterium atom. One proton is the nucleus of a hydrogen atom. A deuterium atom is "heavy hydrogen" which is composed of one proton and one neutron. The reaction is
where the symbols stand for Hydrogen with one proton (1H), hydrogen with a proton and a neutron (2H), also known as deuterium, a positron (e+) which is the positively charged antimatter form of an electron, and a neutrino. The proton and the neutron are both baryons with a baryon number of +1. Hence the left hand side has a total baryon number of +2 as does the right hand side. The total number of baryons remains unchanged in this reaction.