A flat-twin is a two cylinder internal combustion engine where the cylinders are arranged in a flat configuration.
This geometry is probably the best regarding vibrations for a two cylinder engine.
Most of the flat-twin engines built are air cooled.
Flat-twins were used on motorcycles, especially BMWs. The main advantage of this geometry in motorcycles is the superior cooling of the cylinders. The engine itself will also allow a low center of gravity but this advantage is in part balanced by the fact that the engine should be placed rather high in the motorcycle to prevent the cylinders from touching the ground in turns. In practice a longitudinal V-twin like on Moto-Guzzi will give a lower center of gravity but more vibrations and a less efficient cooling.
In automobiles, the flat-twin was popular on small French cars after World War II. Panhard built flat-twin engines which were used in competition on DB and CD cars. These engines were known to have a very high power-to-weight ratio despite their rather simple conception. The Citroën 2CV was also powered by a flat-twin. Despite the fact that the 2CV engine was built with low cost and reliability in mind the last versions of this engine were relatively powerful (giving around 30 hp for 602cc) considering their small cubic capacity.