The principle of lepton number conservation in particle physics asserts that the total number of leptons within a system remains constant throughout any interaction.

Leptons are a category of fundamental particles that include electrons, muons, and neutrinos. The conservation of lepton number is a crucial tenet in particle physics, which dictates that the aggregate number of leptons in a system must not change. This implies that in any particle interaction, the total count of leptons before the interaction must equal the total count after the interaction.

For instance, consider the process of beta decay, where a neutron transforms into a proton, an electron, and an antineutrino. In this interaction, the total number of leptons is conserved. While the neutron itself is not a lepton, it is converted into a proton, which is also not a lepton. However, the emission of an electron and an antineutrino introduces two leptons into the system. Thus, the total lepton count remains unchanged before and after the decay.

Furthermore, the conservation of lepton number is intrinsically linked to the conservation of electric charge, which is another foundational principle in particle physics. Since leptons possess charges of either $-1$ or $0$, the overall number of leptons in a system correlates with its total electric charge. Consequently, any interaction that contravenes lepton number conservation would also lead to a violation of electric charge conservation.