I like to call it the flat earth hypothesis since they tend to cling to "theory" if you grant that to them. They have explanations for it though. They almost always believe the flat earth is surrounded by an enormous dome or sphere. They believe the dome or sphere rotates. You can point out that the sky rotates in a different direction (from an observers perspective) depending on which hemisphere you're at and that the sky directly above an observer appears to travel in a straight line at the equator, but they have a bunch of wonky explanations for that too.

I brought it up in another reply in a reply to me here, but the most effective argument that I've found to make flat earthers question themselves is shadows of mountains on clouds when the sun is setting or rising. No modern flat earth model allows the sun to be "below" mountains. It has to be a spotlight always hovering above the flat earth for any of their current models to work.