Lepton number is a conserved quantity in particle physics that helps classify particles. It is defined as a number assigned to leptons, which are a type of elementary particle that includes electrons, muons, and neutrinos. Each lepton has a lepton number of +1, while their antiparticles, such as positrons and antineutrinos, have a lepton number of -1. In any physical process, the total lepton number before and after must remain the same. This conservation law is important in understanding interactions in particle physics, such as those occurring in beta decay and other weak interactions.