Definition
A jet engine in which a large fan at the front, driven by the engine's turbine, accelerates a mass of air. Part of that air passes through the engine's core (where it is compressed, mixed with fuel, ignited, and expelled through the turbine), while a larger portion bypasses the core and flows around it. Thrust is produced by the combination of the bypass airflow and the hot exhaust gases.
Plain English
A jet engine with a big fan at the front. Some of the air the fan moves goes through the hot core of the engine, but most of it goes around the outside. Both streams push the aircraft forward.
Context Anchor
Seen in aircraft performance discussions, especially when comparing takeoff performance for airplanes powered by different types of engines.
Derivation
From 'turbo' (turbine, the spinning bladed wheel that drives the engine) and 'fan' (the large bladed wheel at the front). The name describes exactly what it is: a turbine-driven fan added to a jet engine to move a much larger mass of air more efficiently.
Why Pilots Care
Higher bypass ratios improve fuel efficiency and reduce noise, directly affecting calculated takeoff distance and climb performance.
Intuition Check
A turbo fan engine is not just a regular fan added to an engine. It is a jet engine in which the turbine drives a large fan that produces a major part of the thrust.
Example Sentence 1
Most modern airliners are powered by turbofan engines because they offer a good balance of thrust, fuel efficiency, and noise levels.
Example Sentence 2
On a hot day the pilot checks the turbofan engine's reduced thrust output before deciding on a reduced takeoff weight.