What is Engine Pressure Ratio? I kind of know what it is but I don't fully understand it. Wikipedia says,


  the engine pressure ratio (EPR) is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.


I know where the entry of the compressor and exit of the propelling nozzle but I don't get what they mean by pressure. Do they mean air pressure?
The wiki quote refers to total pressure at intake and outlet. Total pressure is defined as static pressure $p_s$ and dynamic pressure $p_d$ which for incompressible flow = $\frac{1}{2} \cdot \rho \cdot V^2$.

So the Engine Pressure Ratio is 
$$\frac {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{exhaust}} {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{freeStream}}$$

The turbofan exhaust gas is a mixture of the jet exhaust and the bypass air exhaust.