http://en.wikipedia.org/wiki/Coupé
As the reliable source above goes...
The International Standard ISO 3833-1977 defines a coupé as having a closed body, usually with limited rear volume, a fixed roof of which a portion may be openable, at least two seats in at least one row, two side doors and possibly a rear opening, and at least two side windows.[5]
Alternatively, a coupé is often distinguished from a two-door sedan by the lack of a B pillar to support the roof. Sedans have an A pillar forward at the windscreen, a B pillar aft of the door, and a C pillar defining the aftermost roof support at the rear window. Thus with all side-windows down, a coupé would appear windowless from the A to the C pillars. These fixed-roof models are described as a hardtop or pillarless coupé. Though, to confuse things even further, there are many hardtop/pillarless two- and four-door sedans. Targa top, or just 'T'-top models are a variation on the convertible design, where the roof centre section can be removed, in one or two sections, leaving the rest of the roof in place. Yet another variation on the convertible or drop-head coupé is the fully retractable hardtop. In this form the car has all the advantages of fixed-head vehicle but, at the touch of a button, the entire roof lifts off, folds and stows away in the trunk (boot). Though retractables were tried many years ago by Peugeot, in Europe and Ford, in the US, with the Fairlaine Skyliner, it is only in the 21st century that there has been an explosion in the popularity of this bodystyle.