A Flat-Eight or Horizontally-Opposed Eight is an internal combustion engine in flat configuration, having 8 cylinders.
A well-known engine of this type is the Porsche air-cooled flat-8 engine. It was introduced as a 1.5 litre engine for Formula One, and grew to a 3 litre version used in the Porsche 908.
Another example is the Australian-made Jabiru Aircraft 5100. This is a 5.1 litre, air-cooled aircraft engine.
Continental and Lycoming have both produced flat-8 aircraft engines. The Lycoming IO-720 and Continental IO-720 series of engines have been in production for several decades.