Definition
In a turbofan engine, the ratio of the mass of air that flows around the engine core (bypass air) to the mass of air that flows through the core (where it is mixed with fuel and burned). A high-bypass engine moves much more air around the core than through it; a low-bypass engine moves a smaller proportion around the core.
Plain English
It compares how much air the big front fan pushes around the outside of the engine to how much air goes through the hot, burning center. A high number means most of the air goes around the outside.
Context Anchor
Seen in jet engine basics when comparing turbofan engine design, efficiency, noise, and thrust production.
Derivation
Bypass' simply means to go around rather than through. The 'ratio' part compares the two airflows. The name describes exactly what is being measured: how much air bypasses the core compared to how much goes through it.
Why Pilots Care
Higher bypass ratios produce more thrust with less fuel and lower noise, which directly affects range, operating costs, and environmental performance.
Grounding Statement
Picture the air entering a turbofan: some goes through the hot middle of the engine, but a larger amount may be pushed around that middle by the fan.
Intuition Check
Bypass ratio is not a percentage of total engine air. It is a comparison of air going around the core to air going through the core.
Example Sentence 1
Modern airliners use high-bypass ratio engines because they are quieter and burn less fuel than older designs.
Example Sentence 2
A higher bypass ratio improves fuel economy because most of the thrust comes from the cooler, slower-moving bypass air.