When I was a kid, coupes were 2 door, 2 seats. In the '70s (I think), Chevy started calling their 2 door Impalas a "coupe". Of course, Cadillac has been calling their 2 door land yachts "coupe" since the '50s.

I would think anything with a usable back seat would not be a coupe, except for the rumble seat Model A.