Split horizontally or vertically – which one is which?

Solution 1:

When you split some object horizontally you cut it across its long axis. To cut it vertically you section it through its short axis. That's the way I understand it.

This is a horizontal split.

Solution 2:

'Horizontal' means 'relating to the horizon', so strictly speaking whether a split is vertical or horizontal depends on its orientation relative to the ground. Or less strictly, 'horizontal' is whatever the observer considers to be left/right rather than up/down.

Whether that's the long or short axis has no bearing (which is good for the OP's terminal window example, otherwise terminology would need to change based on the window's proportions!)

I do agree with @Centaurus that the direction of the split is the important thing though (like if there is a split in a piece of wood).

Horizontal split = the dividing line goes from left to right, and remaining parts are stacked on top of one another.

But it's clearly not always entirely obvious to a reader, so it's worth trying to find another way to communicate the intended orientation where possible.