bypass ratio

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|
     |---------------------
     |
  ----------------