Plain English Meaning

Mach is how fast you are going compared to the speed of sound around you. Mach 1 is the speed of sound. Less than 1 is slower than sound, more than 1 is faster than sound.

Full Definition

A unit expressing the speed of an aircraft as a ratio to the speed of sound in the surrounding air. Mach 1.0 means the aircraft is moving at exactly the local speed of sound; Mach 0.8 means it is moving at 80 percent of that speed.

Context Anchor

Seen in high-altitude and high-speed flight, especially on jet aircraft speed limits and performance discussions.

Derivation

Named after Ernst Mach, an Austrian physicist who studied how objects behave at high speeds and how shock waves form. The unit was named in his honor because his work laid the groundwork for understanding flight near and beyond the speed of sound.