What is the Mach number an indicator of?

The Mach number is a dimensionless quantity that represents the ratio of an object's speed to the speed of sound in the surrounding medium. More specifically, it is defined as the ratio of true airspeed (TAS) to the local speed of sound. This measurement is crucial in aerodynamics and aviation, as it helps to understand the behavior of an aircraft as it approaches or exceeds the speed of sound.

When the Mach number is less than 1, the aircraft is traveling subsonically; as it approaches 1, it nears the speed of sound. Mach numbers greater than 1 indicate supersonic flight, while numbers significantly greater than 1 suggest hypersonic speeds. Understanding this relationship allows pilots and engineers to analyze performance characteristics, pressure changes, and aerodynamic forces acting on the aircraft.

The other choices refer to different aspects of airspeed measurement, but they do not accurately define the Mach number as it pertains to the speed of sound. Therefore, the correct interpretation of the Mach number is its direct relation to the local speed of sound through its definition involving true airspeed.