Plain English Meaning

Air moving fast enough that it squeezes and stretches noticeably as it flows, and any heating or cooling inside that flow comes from the squeezing itself, not from outside heat. This is the model used to describe how air behaves around a fast-moving aircraft.

Full Definition

A flow of air in which density changes significantly with pressure (compressible) and no heat is exchanged with the surroundings (adiabatic). At higher airspeeds, especially approaching and above roughly Mach 0.3, air can no longer be treated as having constant density, and any temperature changes within the flow come from the air being compressed or expanded rather than from heat flowing in or out.

Context Anchor

Seen in performance-speed and high-speed airflow discussions, especially when the explains pressure changes and behavior at higher speeds.