An engine in which the fuel is combusted separately from the chamber in which the resulting energy is converted into mechanical force. The most obvious example of this is a jet engine, since the fuel is combusted in a chamber separate from the drive turbine. Another example would be a steam engine, such as the old coal-powered monstrosities that drove the trains of old: the fire boiled water, which turned into steam, and that in turn was routed to the pistons driving the train. Since the fire itself isn't inside the pistons, but pretty far removed, the train qualifies as an external-combustion engine.

Some Scramjets are classified as external-combustion by virtue of their configuration alone: they are little more than air funnels; once you're going fast enough (well over Mach 1), the air is hot enough at one point in the funnel that all you have to do is dump fuel at that point to get combustion.