I remember having an argument once with another guitar player about ghost notes. We never agreed and I beleive both of us were right.

A ghost note in a tab can be either a note that is played softer than the other notes or it can also be an optional note. A note that it wouldn't matter if you played it or not.

You still find these two definitions all over the net. I'm curious has to where music schools stand on this.

Either way, if I saw a (0) in a tab, it would tell me that I could play the open string ( 0 not being a fret but rather the open string ) if I wanted to or not. It would also mean that I can play it but it must sound softer than the other notes that are note Ghosted.

Like I said, I've seen both definitions for ghost notes over the years.