What is bypass ratio and why does it matter for turbofan performance?

Bypass ratio (BPR) is the ratio of mass flow through the fan bypass duct to mass flow through the engine core. Higher BPR means more thrust comes from the low-velocity bypass flow, improving propulsive efficiency at subsonic speeds because thrust = mass flow x velocity change, and accelerating more air to a lower velocity is more efficient. Modern commercial engines have BPR of 10-12:1, achieving 25% better fuel efficiency than older 5:1 designs. However, high BPR requires larger fans with weight, drag, and installation challenges. Military engines use low BPR (0.3-1) for compact size and supersonic performance.