Plain English Meaning
The number on the Machmeter that tells the pilot how fast the airplane is flying compared to the speed of sound around it.
Full Definition
The Mach number reading shown on the aircraft's Machmeter, expressing the airplane's true airspeed as a ratio of the local speed of sound at the current altitude and temperature. It is uncorrected for instrument or position errors.
Context Anchor
Seen in high-speed and high-altitude instrument flying, especially when the aircraft is operated by Mach number instead of ordinary airspeed alone.