A jet engine is a reaction engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's third law of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets, but in common usage, the term generally refers to a gas turbine Brayton cycle engine, an engine with a rotary compressor powered by a turbine, with the leftover power providing thrust. Jet engines are so familiar to the modern world that gas turbines are sometimes mistakenly referred to as a particular application of a jet engine, rather than the other way around. Most jet engines are internal combustion engines but non combusting forms exist also.

Jet engines are primarily used by jet aircraft for long distance travel. The early jet aircraft used turbojet engines which were inefficient. Modern jet aircraft usually use high-bypass turbofan engines which help give high speeds as well as, over long distances, better fuel efficiency than many other forms of transport. About 7.2% of the world's oil was ultimately consumed by jet engines in 2004 In 2007, the cost of jet fuel, while highly variable from one airline to another, averaged 26.5% of total operating costs, making it the single largest operating expense for most airlines