Plain English Meaning

An aircraft engine where the cylinders lie flat and face each other in two rows, one row on each side of the crankshaft, instead of being arranged in a V or a circle.

Full Definition

A reciprocating piston engine arrangement in which the cylinders are mounted in two rows, lying flat and pointing directly away from each other on opposite sides of a central crankshaft. The pistons in each opposing pair move toward and away from the crankshaft simultaneously, which gives the engine a smooth, balanced running action and a low, flat profile that fits well in light aircraft cowlings.

Context Anchor

Seen in discussions of common piston aircraft engine types, especially on many training airplanes and light general aviation aircraft.