Plain English Meaning
How fast the aircraft is moving compared to the speed of sound around it, expressed as a fraction or multiple of that speed.
Full Definition
The ratio of the true airspeed of an aircraft to the speed of sound in the surrounding air. Mach 1 equals the local speed of sound; Mach 0.80 means the aircraft is moving at 80% of that speed. Because the speed of sound varies with air temperature, the same Mach number can correspond to different true airspeeds at different altitudes.
Context Anchor
Seen in high-altitude flight, jet operations, aircraft limitations, and air traffic control speed assignments.