As opposed to any movement limited to one country, whether it be Israel, Ireland, et cetera.
I'm genuinely surprised you've not heard of this, unless you think I'm just talking about some Hamas conquest idea, but a previous post specified pro-two state solution. I'm talking about the one state solution, which is the abolition of Israel (not synonymous with the expulsion of Jews, but it does mean the end of Israel as the replacement would be some type of single Israeli-Palestinian state) which has been promoted for decades. Obviously, Netanyahu's annexation would also be a type of one-state solution, but what it means is a single state with equal rights for both peoples. In some formulations it's explicitly binational with clearly defined group rights (as in NI). It's controversial, but not extremist; it's something extensively discussed in academic circles. And this is why the accusations of "Zionist" like it's "Nazi" are so controversial: to many Jews, Zionism is simply the right of Israel to exist and not a defence of any government or its actions.
It's the position of groups such as Jewish Voice for Peace which is why that group is controversial within the Jewish community. It's not new. Edward Said famously supported it. Politicians and the UN officially hew to the two-state solution line because, at the very least, it would be irresponsible for any outside party to unilaterally change the terms of a settlement.
Palestine Solidarity Campaign does not officially take a stance (though its stance on removing all settler colonialism and full realisation of the right to return is contrary to Oslo's principles) but within PSC the one-state solution enjoys increasing support.
I run in left-ish Jewish circles and the two state vs. one state debate is a constant topic. It's also been key in the debate over the IHRA definition of antisemitism, as one state solution advocacy can be viewed as denial of Israel's right to exist and thus in contravention of this definition.