2.1: Lepton
Lepton is a term introduced by Friedrich Rinne in his book Crystals and the Fine-Structure of Matter (London: Methuen & Co., 1924, English translation by S. W. Stiles) to indicate electrons, atoms, ions, radicals, and molecules building the unit formula of a crystalline compound. The crystal structure of a solid was sometimes called "leptonic structure".
Today, the term lepton is used to indicate spin-1/2 particles and Rinne's use is only of historical interest.