Plain English Meaning
The aircraft's speed, expressed as a fraction of the speed of sound, at which the air flowing over some part of the wing first goes supersonic — usually over the thickest part of the wing, where the air has to speed up to get around it.
Full Definition
The lowest Mach number at which airflow over any part of an aircraft first reaches the speed of sound, even though the aircraft itself is still flying below the speed of sound. Also called critical Mach number (M Crit or MCRIT).
Context Anchor
Seen in high-speed flight discussions, especially when learning why an aircraft can have speed-related problems before it actually reaches Mach 1.
Derivation
“Mach” comes from Ernst Mach, the Austrian physicist who studied the behavior of objects moving through air at high speeds. A Mach number is the of an object's speed to the speed of sound. “Crit” is short for “” — meaning the where something important changes. So Mach Crit is the critical Mach number where airflow behavior begins to change in important ways.