A flat engine or boxer engine or horizontally opposed engine is a type of engine where the pistons lie horizontally opposed, with pairs of cylinders on the left and the right, as opposed to most modern engines where the alternating pistons are positioned in a "V"-angle. Some experts distinguish between V-at-180° engines and real boxer engines. The real boxer has one crank pin per piston while in the V-at-180° engine two pistons share the same crank pin. Real boxer engines can run perfectly smooth and free of vibrations with a four-stroke cycle, regardless of number of cylinders. The benefit of using a boxer engine versus a V engine is that it provides perfect balance because each piston's movement is exactly counterbalanced by the corresponding piston movement of the opposite side. Boxer-engined automobiles also generally have a lower center of gravity, giving better stability and control. However, boxer engines are also wider than normal engines and are more expensive to build than traditional engines and, thus, are not widely used by automobile manufacturers. The flat configuration fits very well with air cooling and aircraft engines. Air-cooled designs such as in the VW Beetle used a Flat-4, as did the Porsche 356 and 912. Both the older and newer models of the 911 use a Flat-6, while Subaru use the flat-4 design extensively. Ferrari use a Flat-12 on the Testarossa. Air-cooled Flat-twin engines were used by Citroën in their model 2CV and its derivatives, while the Citroën GS used a flat-4, and a Flat-6 was proposed for the Citroën DS but rejected due to impractability. BMW uses a flat-twin in many of their motorbikes. Also try: http://en.wikipedia.org/wiki/Boxer_engine Also see: http://www.answerbag.com/q_view.php/5763 for additional engine types/info. by another user (jervinator).

I cannot verify this answer but when I was a teenager i heard that the BMW motorcylcle "Boxer" engine was called that because the valve covers resembled boxing gloves.

Boxer engine is such that the pistons are "boxing each other" =opposed piston engine. That means that two pistons move against each other and they have common compression chamber. 4 cylinder engine has 2 cranks. This term "boxer engine" has later been applied rather incorrectly to the "flat 4", which was used by VW, Subaru, etc.

originally, an engine whose bore size and stroke was considered square or "boxed" as opposed to being "over" square or "under" square. The old BMW bikes had a boxed motor, the term Boxer was applied to those engines

The boxer engine, also known as a horizontally opposed engine, in which the corresponding pistons reach top dead centre simultaneously, thus balancing each other with respect to momentum. Flat engines with four or fewer cylinders are most commonly boxer engines.

A boxer engine? because it causes barfights when some drunk insists know one knows the right answer but him.