Plain English Meaning

Flying fast enough that the air itself starts to behave differently — it begins to bunch up and compress against the airplane, which changes how the airplane flies and handles.

Full Definition

Flight at speeds where the compressibility of air becomes a significant factor in aerodynamic behavior, typically at or near the speed of sound. At these speeds, airflow over portions of the airplane can approach or exceed Mach 1, producing shock waves and changes in lift, drag, and control response that do not occur at lower speeds.

Context Anchor

Seen in airplane handling, performance, and transition training when moving from slower training airplanes to airplanes that fly faster or have more demanding speed limits.