Front, front mid, rear mid / mid, rear - Chassis definitions
Clarifications for newcomers:
Front Engine: Engine and usually transmission are placed forward of the passenger compartment. Most often the engine sits over the front axle line. Note: many front wheel drive engines are placed ahead of the front axle line.
Front Mid Engine: The Engine is placed mostly or completely behind the front axle line, but ahead of the passenger compartment.
Rear Mid Engine / Mid Engine: The engine is placed ahead of the differential and behind the passenger compartment. The transmission might be forward of the differential, integral to the differential, or rearward of the differential.
Longitudinal Mid Engine: The engine is placed length ways - front to back - ahead of the differential.
Transverse Mid Engine: The engine is placed with the front of the engine facing one of the sides of the car. Most frequently a belt is used to drive a transaxle (like a front wheel drive). This enables a compact layout but power / torque is usually limited due to the limits of existing transaxles.
Rear Engine: Engine is placed behind the differential. (Porshe 911, Corvair, original VW Beetle).