# What do ε₀ and V mean in a Pourbaix diagram?

From Wikipedia:

What does this graph mean?

What is $$\varepsilon$$? It looks like $$\varepsilon_0$$, like the vacuum permittivity, but what does this have to do with the polymorph and/or isomers of ferrite or hematite?

And what is the V or 'five' in square brackets after the $$\varepsilon_0$$?

The abscissa is the pH, the ordinate the reduction potential (with respect to the standard hydrogen electrode): thus, it is not a question of the size epsilon but of the potential $$E$$ which has been a little "stylized" as $$\mathcal{E}$$, which is expressed in volts (hence the symbol V).
This is a Pourbaix Diagram (I assume this is where you got this figure). While it's not a common notation as far as I'm aware, $$\epsilon_0$$ is being used to represent the reduction potential in volts (V). As mentioned at the top of the Wikipedia page, this is basically a phase diagram, that tells you the ranges of voltage/pH for which a particular species of (in this case) iron is most thermodynamically favorable. The dividing lines coincide with an even mixture of the surrounding species.