I don’t do polemical definitions of “revisionist.” I’m using it in a neutral sense: revisionists are historians who critically re-examine common received wisdom and authoritative accounts about history, and criticize or rejecting the “official” or authoritative understanding of the events.
Whether or not this project is really worthwhile depends on what’s being rejected and what the evidence for the rejecting is. Since I tend to think that official/governmental accounts of history tend to be a pack of distortions, fudging, and self-serving lies, I tend be pretty positive on revisionism, so long as the revisionist in question is herself serious and honest. Zinn’s a good example; I’d also consider somebody like J.R. Hummel or Bob Higgs an example of good honest revisionism. Of course, there are other revisionists out there who are ignorant, stupid or dishonest — take David Irving (please!). But the problem with them isn’t that they’re revisionists. It’s that they’re idiots or charlatans.