What exactly is the meaning of a degenerate partial differential equation?

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I'm trying to understand the meaning of a degenerate partial differential equation. I've only found the definition by Giga in Surface Evolution equation but I wanted to ask if somebody knows another reference to this type of partial differential equations. And what does it mean if a partial differential equation is degenerate not in the sense of the definition but in the geometrical sense? What are the problems that we have to face by analyzibg these pde? Maybe there are good references to these questions too.