The Specific Thrust of an engine is the ratio of the thrust, T, to the engine core mass flow, ma. This is commonly used as one of the performance criteria by which engines are assessed. For a simple turbojet, the specific thrust is represented by the equation:
T/ma = (1+f)*Ue - U
where Ue is the jet exhaust velocity and U is the flightspeed.
For a turbofan engine, the specific thrust can be written as:
T/ma = (1+f)*Ue + B*Uef - (1+B)*U
where B is the bypass ratio of the engine and Uef is the bypass flow exit velocity.