The definition of a coupe is one of those questions that if you ask five people for their definition you’ll get five different answers.
Here is Hemmings Classic Car Guide definition:
http://www.hemmings.com/hcc/stories/2007/03/01/hmn_feature6.html
Here is the Hemmings Classic Car Guide definition of a coupe:
• Coupe: Any two-door other than a Two-Door Sedan, smaller than a related four-door in the same model line. If there is no sedan in that model's lineup, the two-door is a coupe and cannot be a Two-Door Sedan. All two-door two-seaters with a solid roof are coupes. Many manufacturers have referred to qualifying convertibles and retractable-roof cars as Convertible Coupes when the tops were up.
• Hatchback Coupe: Any coupe with a luggage compartment contiguous with the passenger compartment and accessed via a large rear liftgate.
• Sports Coupe: Any coupe having no B-pillar, often a fastback; also called Hardtop Coupe, Two-Door Hardtop, Berlinetta, Berlinette.
• Business Coupe: A two-door with a single row of seating and space between the seatback and the end of the passenger compartment, as for a salesman's sample case.
• Opera Coupe: A two-door with a single row of permanent seating and folding occasional or jump seats as the second row.
• Club Coupe: A two-door with two rows of seating and a trunk, smaller than a sedan in the same lineup for comparison, generally having a fixed B-pillar, and a rear seat farther forward than in a comparable sedan. Also called Close-Coupled Coupe.
• Sedanet (Sedanette): A fastback Club Coupe; also called Aero Coupe, may or may not be a Hatchback Coupe. Originally a sedan, which resembled a Coupe.
• Two Door Sedan: A two-door with two rows of seating, having the same or nearly the same room and seating configuration as a sedan in the same lineup, usually with a profile identical to a sedan. Also called Business Sedan (if lacking rear seat), Brougham (also applied to four-doors), Brougham Coupe, Coach, Club Sedan, Victoria (also applied to smaller coupes).
However, Keyboard is absolutely right too.
The definition he presented came directly from the Society of American Engineers (SAE). While defining a coupe based on its interior volume seems arbitrary, it is recognized by almost everyone knowledgeable in the car industry. It always seemed odd to me that America made this definition but the foreign manufactures actually use it. Based on the SAE definition, Mercedes calls their 4-door CLA a coupe and VW calls their 4-door CC a “Comfort Coupe.” Even though Mazda marketed their RX-8 as a four door car, it was still called a coupe.