There was a technical definition dating back to 1977, for a Coupe it was defining leg- and headroom in the rear, even the number of doors and windows and more ISO 3833-1977

I once had the link to a free download, can't find it anymore. Volvo had two- and four door sedans, while the 262C was a coupe. Then there were e.g. two-door BMW sedans which were sold as coupes. Also, four-door coupes have been introduced, so it is quite obsolete anyway

I think another "official" difference is the seating: 4 adults in a 2-door sedan vs 2 adults and (sometimes) 1 or 2 children in the coupé (e.g. the coupés named "2+2").
For example the classic VW Beetle and BMW E30 are 2-door sedan, even if the VW has a sloped rear unlike the BMW that has a classic sedan shape but just two doors.

Very interesting question... i scratched my head a few times on this too...

Once upon a time, there used to be a difference. 2-door sedans used to have solid B-pillars and were more upright, like the 4-door sedans. Coupes tended to have more sloped roofs and by the 1950s, often hardtops (without B-pillars). The coupes generally were considered more high-end and were more expensive. But by the late 1970s, most any car with only two doors was called a coupe. Europeans held out a little longer. Mainly marketing killed the 2-door sedan, as "coupes" sold and "2-door sedans" were for grandmas.

I've noticed on this website, that many coupes are actually listed as "two-door sedans" and I'm wondering just what the difference is? I've performed Google searches on this, but can't find results telling of a difference between the two terms at all. I see in the comments under many cars on this website that many of these cars are "not coupes, but two-door sedans". Is it the car's seating arrangement, windows, or what?

I'm confused...