The term 'lepton' is a type of subatomic particle, and the crossword solution for it is 'TAU' (3 letters). Leptons are one of the two basic components of matter, along with quarks. They are fundamental particles that do not experience the strong nuclear force, and they are believed to be massless. TAU leptons are the heaviest of the leptons, and they have a mass of 1776.82 MeV/c2. They are also unstable and decay quickly into other particles.