What distinguishes a “sedan” from other body styles differs somewhat according to various sources. Some people will claim that the absence of a mid-body pillar is key to make a car a “coupe” as opposed to a two door “sedan”. Others think it corresponds strictly to 4 vs. 2 doors. However, you will find “two door sedans” referred to various places.
From “Car and Driver”'s glossary:
Their definition of “coupe” references the same standard, and says “fixed roof” with < 33 cu. ft. So, if you want to believe the SAE, it is partially a measurement of interior size.
To hell with the SAE. Back in my day, a sedan was defined as a car with a B-pillar (the mid-body pillar.) Without a B-pillar, the coupe could have a lower-slung roof line, thereby giving it a more sporty look. It was possible to have a 2-door sedan, as well as a 4-door coupe.
As a side note: I read an article in the NY Daily News yesterday (so it must be true) that the term station wagon originated when hotels would convert the rear of cars to carry luggage when they picked up guests at the train station.
The auto industry eventually caught on to the idea and designed their own version.