In a turbofan engine the bypass ratio is the ratio of how much air passes around the engine compared to how much enters the compressor section. the Pratt & Whitney PW4000 112 inch has a 6.4 to 1 bypass ratio. This makes newer engines more fuel efficient and cleaner burning because most of the engine's thrust is created by the fan instead of the turbine. A simple diagram:
Engine
----------------
| bypass
F|---------------------
A|intake
N|
|---------------------
|
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