You should see my other comment above, because I talk about some of this there.
If the extremes are campus radicals and the alt-right, then Bernie is not that far left. He is also more conciliatory than party-line Democrats or Republicans.
I would also argue that what "moderate" (or perhaps more accurately "independent") really means in the modern context is not as simple as falling squarely between Democrat and Republican values on any given issue. I have views that go both left and right, at least as typically defined by people who disagree with them. I call my views "moderate" because it is easier to understand than saying that they are part of a coherent worldview that is not defined by our simplistic binary party system.
I don't think this is a term that should really need to be defined - it's not as though everyone who identifies as moderate is from Futurama's Neutral Planet.