Page "Aerodynamics" Paragraph 28

All of these effects are often mentioned in conjunction with the term " compressibility ", but in a manner of speaking, they are incorrectly used.

From a strictly aerodynamic point of view, the term should refer only to those side-effects arising as a result of the changes in airflow from an incompressible fluid ( similar in effect to water ) to a compressible fluid ( acting as a gas ) as the speed of sound is approached.

There are two effects in particular, wave drag and critical mach.