When we raise the pressure of the air, the temperature increases and therefore so does the entropy of the fluid.
The entropy is a measure of energy loss. A real compressor requires more work to raise the pressure of the air to the same outlet pressure than a theoretical compressor.
But the compression efficiency isn‘t the whole story. A compressor depends on the airend‘s efficiency and the compression chamber but its total energy consumption depends also on several losses and efficiencies of the components included in the package. (See Figure 5)
In reality there needs to be one measure to compare efficiency of one compressor to another.