Longman Dictionary of Contemporary English
n. 1 an engine that obtains thrust by taking in air at the front, using it to burn fuel, then ejecting the hot combustion products at the rear through a propulsive nozzle 2 any engine propelled by expelling a high speed fluid jet (jet propulsion), such as a rocket, turbojet, turbofan, ramjet etc.
n. a gas turbine produces a stream of hot gas that propels a jet plane by reaction propulsion
A jet engine is a reaction engine discharging a fast-moving jet that generates thrust by jet propulsion. This broad definition includes turbojets, turbofans, rocket engines, ramjets, and pulse jets. In general, jet engines are combustion engines.
In common parlance, the term jet engine loosely refers to an internal combustion airbreathing jet engine. These typically feature a rotating air compressor powered by a turbine, with the leftover power providing thrust via a propelling nozzle — this process is known as the Brayton thermodynamic cycle. Jet aircraft use such engines for long-distance travel. Early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use more complex high-bypass turbofan engines. These engines offer high speed and greater fuel efficiency than piston and propeller aeroengines over long distances.
The thrust of a typical jetliner engine went from ( de Havilland Ghost turbojet ) in the 1950s to ( General Electric GE90 turbofan) in the 1990s, and their reliability went from 40 in-flight shutdowns per 100,000 engine flight hours to less than one in the late 1990s. This, combined with greatly decreased fuel consumption, permitted routine transatlantic flight by twin-engined airliners by the turn of the century, where before a similar journey would have required multiple fuel stops.