The word is French for cut, and it began as a carriage body shortened behind the driver. That is still the useful test. A coupe gives up rear doors, rear-seat space, or both, and spends what it saves on proportion. Roof shape is a separate question and always has been: a coupe can also be a fastback, and most are, but the two words answer different things.
Marketing has since stretched the term across four-door cars and SUVs, which is why the original sense is worth holding somewhere. The cars under this tag still meet it. The Honda Prelude is 4,522 mm long and 1,355 mm tall. The Nissan Z keeps a front engine and rear drive in a two-seat body. The Denza Z, 4,780 mm long and 1,350 mm tall, is the same layout rebuilt around a battery. No mechanical recipe connects them. The common decision is where the interior volume goes, and it goes to the two people in front.
