Excellent help here.

By the way, "flathead" does not refer to the kind of screwdriver end that you turn the screw with. It refers to the way the head of the screw interfaces with the surface you're mounting in. A flathead screw is flat across. For comparison of the concept, look at a round head screw. The head comes up from the mounting surface and there's almost a half sphered of metal there. Its finish is round. An oval head has a finish shape between flathead and round head.

What you're talking about is variously called a slot drive. As google says elsewhere, "Slot screw drives have a single horizontal indentation (the slot) in the fastener head and is driven by a "common blade" or flat-bladed screwdriver."

Don't worry about this too too much. It's a common error.