What is bypass ratio in gas turbine engine?

bypass ratio im an early singlr or twin-spool bypass engine is the ratio of cool air mass flow passed through the bypass duct to the air mass flow passed the high pressure system.

Bypass ratio for the fan ducted bypass engine is the ratio of total air mass flow through the air fan stage to the air mass flow that passes through the turbine section/high pressure(engine core) system.A high bypass ratio i.e 5:1 is usually common with ducted fan engine.