An axial flow compressor is a dynamic (turbo) machine used in gas turbines, jet engines and industrial superchargers, where air flows parallel to the shaft axis and is compressed by successive rows of rotating and stationary blades. Because it is a continuous-flow rotodynamic machine (not a positive-displacement one), the single most important measure of how well it performs is how closely the real compression approaches the ideal reversible-adiabatic process.

That measure is the isentropic efficiency (also called adiabatic efficiency), defined for a compressor as the ratio of the ideal work needed to raise the gas to the delivery pressure to the actual work supplied:

• η_isen = (isentropic work of compression) ÷ (actual work of compression) = (h_2s − h₁) ÷ (h₂ − h₁)
• Here h_2s is the outlet enthalpy for an ideal isentropic process and h₂ is the real outlet enthalpy. Because real compression is irreversible (friction, turbulence, shock losses at high speed), the actual temperature rise is always greater than the ideal one, so η_isen < 1.
• In high-speed applications these irreversibilities grow rapidly, so isentropic efficiency directly captures the aerodynamic quality of the machine and the extra power the driver must supply.

The other choices belong to different machines or roles. Volumetric efficiency relates the actual induced volume to the swept volume and is meaningful only for reciprocating (positive-displacement) compressors, not for a continuous-flow axial machine. Thermal efficiency is a heat-engine concept comparing work output to heat supplied — a compressor absorbs work rather than producing it, so this does not apply. Mechanical efficiency only accounts for bearing and transmission friction between the coupling and the rotor; it is a secondary bookkeeping term and says nothing about the aerodynamic quality of the compression itself.