Much like “Zionism” itself, anti-Zionism can have a range of meanings. But a workable core (and others more knowledgeable in this field should feel free to correct me) might be opposition to 1. a Jewish ethnostate whose maintenance as such is predicated on dispossession & oppression of Palestinians.

Thanks. I appreciate the answer. I'd love to think there's some humane way out of this situation - for all involved.

That is absolutely not what Zionism is. For someone who has spent their career studying Jewish life, you've really missed the mark on that and sound more like you're repeating talking points from the BDS movement. Try the Jewish right to self-determination in their ancestral homeland.

Hmm, this is in practice what it is so spot on

For Jewish anti-Zionists, there is usually a second clause: rejecting 1. specifically as a means of Jewish liberation.

No correction needed as far as I'm concerned. 

Genuine question. What do you consider an ethnostate? Do you think that anti-Zionism outright rejects the existence of any state whose intent is to preserve the the Jewish people?