Air entrainment is the phenomenon that occurs when air (or any gas) under pressure is released from a device in such a way that a low pressure is generated in the immediate area of the air (or gas) discharge. Air (or gas) from the surrounding environment is then pulled (or entrained) into the discharged air stream, increasing its volumetric flow rate.
Thus, it is a free air flow. Every cubic foot that’s entrained means that the compressor didn’t have to spend energy compressing that cubic foot.
(See also, ‘Coanda effect’)