The term 'LEPTON SITE' can be solved by the crossword solution 'ATOM', which is a four-letter word. An ATOM is the smallest unit of matter that retains its chemical properties. Atoms are composed of protons, neutrons, and electrons, and they are the building blocks of all matter. The lepton site is a region of an ATOM where leptons, such as electrons, are located. Leptons are particles that interact with other particles through the electromagnetic force.