Definition
A turbine engine in which a portion of the air drawn in by the front fan is ducted around (bypasses) the engine core rather than passing through the combustion chambers and turbine. The bypassed air is accelerated by the fan and rejoins or exits separately from the core exhaust, contributing a significant share of the total thrust. The ratio of bypass airflow to core airflow is called the bypass ratio.
Plain English
A jet engine where a large fan at the front pushes some air through the hot core of the engine and lets the rest flow around the outside. Most of the thrust comes from that air going around the core, which makes the engine quieter and more fuel-efficient than older pure-jet designs.
Context Anchor
Seen in discussions of turbofan engines, jet aircraft performance, thrust, fuel efficiency, and aircraft noise.
Derivation
Bypass means to go around something rather than through it. The term describes exactly what the air does: a portion of it goes around the engine core instead of through it.
Why Pilots Care
Bypass engines deliver better fuel efficiency and lower noise than pure turbojets, which is why nearly all modern airliners use them.
Analogy
Think of water flowing through a pipe with a second path around the middle section. In a bypass engine, some air goes through the center, while some takes the outside path and still helps produce push.
Intuition Check
Bypass does not mean the engine is being skipped or not used. Here, it means some of the air is routed around the engine core while still helping create thrust.
Example Sentence 1
Modern airliners use high-bypass engines because they are quieter and more fuel-efficient than the low-bypass engines used on older jets.
Example Sentence 2
During the walk-around, the pilot checked the fan blades on the bypass engine for damage.