Definition
A turbine engine in which a large fan at the front of the engine drives a significant volume of air around the outside of the engine core, while a smaller portion of that air passes through the core to be burned with fuel. The thrust produced by the bypassed (cool) air is combined with the thrust from the hot exhaust gases to produce total engine thrust.
Plain English
A jet engine with a big fan up front. Some of the air the fan moves goes through the middle of the engine where fuel is burned, and the rest is pushed around the outside. Both streams of air combine to push the airplane forward, which makes the engine more efficient and quieter than a plain jet.
Context Anchor
Seen when comparing turbine engine types, especially on transport aircraft, business jets, and some advanced training aircraft.
Derivation
From 'turbine' (the spinning bladed wheel inside the engine driven by hot exhaust gas) plus 'fan' (the large bladed wheel at the front). The name reflects the design: a turbine drives a fan, and the fan does most of the work of producing thrust.
Why Pilots Care
Turbofans power most commercial airliners because they deliver high thrust with better fuel efficiency and lower noise than pure turbojets.
Analogy
Think of it as a powerful fan being turned by a hot engine inside it. The fan moves a large amount of air, while the inner engine keeps the fan turning and adds more push.
Intuition Check
Do not think the fan is separate from the jet engine. In a turbofan, the fan and the turbine engine work together as one engine.
Example Sentence 1
The Boeing 737 is powered by two turbofan engines mounted under the wings.
Example Sentence 2
Maintenance crews checked the turbofan blades for foreign object damage before the next flight.