From SKYbrary Wiki
A scramjet is a variant of an air breathing jet engine that does not include a rotary compressor; rather, it uses the engine's forward motion and shock wave effect to compress the incoming air. The word "scramjet" is derived from the phrase "supersonic combustion ramjet" and it serves to describe the primary difference between a scramjet engine and a ramjet engine. That difference is that scramjets do not slow the intake airflow to subsonic speeds for combustion, instead igniting the fuel air mixture at supersonic speeds. The supersonic combustion design allows the engine to operate at speeds above the mach 6 limiting speed of a ramjet engine.
Scramjets are still very much in the experimental phase of development and very few have been actually built and flown. To date, the maximum burn duration of these experimental engines is measured in seconds indicating that there is still much developmental work to be done before the engine will have any practical application.