Should I write Steak or Beef Tenderloin on the menu?
I would say anything that is a slice of meat thicker than around 1cm, taken from a larger muscle and (usually) cut across the grain can be considered a 'steak'.
Depending on what part of the tenderlon you have, you could call it either sirloin or rump steak or filet mignon, just 'tenderloin steak' or even plain old 'steak' (see Wikipedia for more info)

I don't think it is misleading or confusing to call it steak on the menu.