In the Klein-Gordon or in the Dirac Lagrangian density, the mass term is quadratic in the field. The other way around, I have heard a quadratic term in a general Lagrangian density be referred to as a mass term. Why is that, and what does that mean exactly?
I also think I understood that Feynman diagrams don't contain vertices that are connected to only two edges, because that would correspond to a quadratic term, which is a mass term and is treated differently. Does that make sense? How?
