The term, longitudinal means "front-to-back" or "top-to-bottom" as opposed to transverse which means "side-to-side".
In automotive engineering, the term, 'longitudinal' refers to an engine in which the crankshaft is oriented along the long axis of the vehicle, front to back. Most rear wheel drive vehicles use this engine orientation, while most front wheel drive vehicles use a side-to-side transverse arrangement.
A common example of a longitudinal engine is the Boxer engine. This is a design where the cylinders are horizontally opposed, or facing each other.
Common cars that share this design are: Alfa Romeo 145/146 (Flat-4), the Subaru Impreza (Flat-6) and the Porsche Boxer .
See also: