A jet engine that uses ram pressure to compress incoming air, and where the airflow throughout the entire engine is at supersonic speeds.
A scramjet (supersonic combustion ramjet) is, like a ramjet, a kind of jet-engine with no moving parts, using solely ram pressure (the compressive force provided by the aircraft's forward motion through the air) for air compression. Unlike a conventional ramjet, which decelerates said air to subsonic speeds before using it to burn fuel, a scramjet's airflow remains supersonic throughout the entire engine. This decreases the problems inherent in dramatically slowing down (and heating up) incoming air which plague ramjets at very high speeds, and, as such, scramjets are the superior option for hypersonic flight, above approximately mach 5. However, having combustion take place in air that is moving at supersonic speeds requires either a very fast-burning fuel (such as hydrogen), very efficient fuel-air mixing, a very long tailpipe, or several (or all) of the above in order to ensure that the fuel finishes burning before it leaves the engine, which poses considerable engineering difficulties, on top of those experienced by any vehicle in hypersonic flight, regardless of propulsion method - namely, the immense aerodynamic drag and shock heating resulting from travel at extremely high speeds. As a result of all of these hard-to-solve problems, scramjets are still very much in their infancy; the first scramjet flew only in 1991, as a payload on a modified surface-to-air missile, while the first vehicle with a scramjet as its primary engine was NASA's X-43A, which made two ten-second flights in 2004, and the all-time record for the longest flight under scramjet power is only 210 seconds (achieved in May 2013).
For more information about scramjets, see Wikipedia.