A fastback is an automotive styling feature, defined by the rear of the car having a single slope from the roof to the tail.
1935 Stout Scarab
1949 Cadillac Series 62 rear
A coupe or coupé is a passenger car with a sloping or truncated rear roofline and two doors.
Infiniti Q60 coupé
1955 Cadillac Coupe de Ville
Example of a coupe carriage
A 1936 Packard One-Twenty business coupe