Horizontal or vertical
A horizontal flip swaps left and right, which is what a mirror does and what most people mean by flipping. It is the fix for selfies that saved mirrored, and a quick way to make a subject face into a layout rather than out of it.
A vertical flip swaps top and bottom. That reads as upside down rather than mirrored, so it is used less often on photographs — its usual job is producing a reflection to place beneath an object.