A semileptonic decay contains both final-state hadrons and leptons, typically through a weak charged current. This distinguishes it from a purely leptonic decay.
Pion beta decay changes a charged pion into a neutral pion and a lepton pair through the weak charged current. The small charged-neutral pion mass gap severely restricts phase space, giving an approximately fifth-power dependence on that gap. It does not have the charged-lepton mass suppression of direct leptonic pion decay.
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Semileptonic decay is a type of particle decay process that involves both hadrons (particles composed of quarks, such as baryons and mesons) and leptons (fundamental particles that do not undergo strong interactions, such as electrons, muons, and neutrinos). In a semileptonic decay, one of the hadrons transforms into another hadron, while simultaneously emitting a lepton and a corresponding antiparticle (usually a neutrino).