A flat-6 is a 6 cylinder configuration of a flat engine or "boxer" engine. The cylinders are laid horizontally, with three on each side. The movement of the pistons in a horizontal engine is all in the same plane, so it creates less vibration than in a V-configuration engine; particularly one with an odd number of cylinders on each side of the engine, like a V6.
Only a few auto makers have used flat-6 engines, among them Porsche and Subaru. Citroën developed one for the DS but abandoned it due to numerous problems. In the 1960s, Chevrolet made the Corvair with flat-6 air-cooled engines.