A state that is both culturally and politically dominated by an ethnic group, and where the government explicitly prioritizes that group above all others. For example, South Africa when controlled by whites wasn't really a white ethnostate - they were politically dominant, but not culturally dominant. Now it is an african ethnostate, because it is culturally and politically dominated. Germany under hitler would have been an ethnostate, Germany now isn't. Israel is, because it is dominated specifically by jewish culture and jewish people.

It has to be to an undeniable degree. For example, I would say that China, despite being pretty xenophobic as a culture, isn't really an ethnostate, or at least not to the same degree - because everyone is treated like shit there. The government doesn't promote an ethnic group, it promotes itself at the expense of anyone it can exert control over. I don't think the US was an ethnostate - while it excluded a specific group or multiple groups, it didn't prioritize one specific group - there was always a confluence of different ethnicities involved, even counting just the european groups at first.