[wfunction]

I thought pH = -log[H+]? where [H+] is the molarity of the hydrogen ion? That doesn't involve water in the definition... what is the definition you're referring to?

Edit regarding your reply asserting water is necessary (I can't reply now due to throttling):

What about all the acid/base reactions that don't involve water? You can't ascribe any pH to either of them in that case?

[Pulcinella]

[H+] is shorthand for [H3O+]. H3O+ (hydronium ion) requires water plus a hydrogen ion from the acid. There aren't actually free H+ ions floating around. They are all attached to water molecules.

[yogipatel]

That's the definition of p[H]. The definition of the "real" pH is the -log of H+ activity. H+ activity is the concentration of H+ * a coefficient. In practice, the coefficient is considered to be 1, however there are cases (such as extreme concentrations) when that approximation isn't valid.

[Pulcinella]

Ah yes I had forgotten about activity. :)