What is the common name for a horizontally opposed engine?

The common name for a horizontally opposed engine is a boxer engine.

In a boxer engine, cylinders face opposite directions on either side of a central crankshaft. Each opposing pair of pistons moves inward and outward together, resembling two boxers punching toward each other. This arrangement gives the engine a low center of gravity and inherently good balance.

“Flat engine” is the broader technical term for the layout, and horizontally opposed engine is another descriptive name. Most flat engines use the boxer configuration, although some flat engines—especially certain flat-twelve designs—use a 180-degree V arrangement instead. In everyday automotive discussion, however, “boxer engine” is the familiar name for the opposed-piston configuration.

The layout has a long history: Karl Benz built the first flat engine in 1897. Volkswagen became strongly associated with air-cooled boxer-fours, Subaru with boxer-fours and boxer-sixes, and Porsche with the flat-six engines used in the 911. The trade-off is greater width than an inline engine.