A boxer engine is a four-stroke engine that has its pistons move simultaneously toward and away from one another on a horizontal plane. Each pair of pistons that lie next to each other has a motion that resembles a boxer throwing his fists, hence the name of the engine.
The horizontal layout of the cylinder offers the benefit of a low center of gravity and a balanced weight. This is desirable for overall stability and vehicle control, minimizing body roll and enhancing handling precision. The side-to-side movement of any two pistons cancels out vibrations produced due to combustion and ignition. Because of this, boxer engines don't need counterweights on the crankshaft or excessive vibration dampening systems, like V-type or inline engines do.
A boxer engine shouldn't be confused with a flat engine; a flat engine is essentially a V-type engine with 180 degrees between the two cylinder banks. This means the flat engine also has its cylinders move in a horizontal plane, but the flat engine uses one crankpin to connect two opposed pistons, while a boxer engine uses only one crankpin per piston. This means that piston pairs in flat engines move together, rather than in and out like a boxer's fists. Because of this, while flat engines also offer a low center of gravity, they don't have the same vibration dampening characteristics as boxer engines.