Yes - it should be "final" as opposed to "finals", although the latter word is commonly used. In layman's language it's when the aircraft is very close in to the airfield during its landing approach, say within a mile or so. "Long Final" is on the approach outside 4 nm but I haven't been able to find a definition for the term "short final" which, again, is in popular use. Pilots and controllers know what it means though. If you are a controller and have three lined up for departure, when someone reports "short final" is when the little window in the side of your head that reads your age starts going round like a fruit machine.