Plain English Meaning
A turbojet is a pure jet engine that makes thrust by squeezing air, burning fuel with it, and shooting the hot exhaust out the back at high speed. The whole airplane is often called a turbojet too.
Full Definition
Turbojets are jet engines in which all of the air drawn in passes through the compressor, combustion chamber, and turbine, with thrust produced entirely by the high-velocity exhaust gases leaving the rear of the engine. Aircraft powered by these engines are commonly called turbojets as well.