A flat-8 or horizontally opposed-8 is an internal combustion engine in flat configuration, having 8 cylinders.
Another example is the Australian-made Jabiru Aircraft 5100. This is a 5.1-litre, air-cooled engine for aircraft use.
Continental and Lycoming have both produced flat-8 aircraft engines as well. The Lycoming IO-720 and Continental IO-720 series of engines have been in production for several decades.