An air-cooled boxer engine is a type of internal combustion engine where the cylinders are arranged horizontally, or "flat," on either side of a central crankshaft. This design allows for a lower center of gravity, improving vehicle stability. The engine is cooled by air flowing over the cylinders, eliminating the need for a liquid cooling system. This engine configuration is commonly found in vehicles like the Volkswagen Beetle and Porsche 911. Its simplicity and compact design make it popular in various applications, including motorcycles and small aircraft, where weight and space are critical factors.