Plain English Meaning

Air flowing past part of the airplane faster than sound travels through that air. Once the flow goes supersonic, it behaves very differently from normal subsonic airflow, and shock waves appear.

Full Definition

Airflow moving faster than the local speed of sound (Mach 1.0). In supersonic flow, pressure changes cannot travel upstream against the airflow, so the air ahead of the airplane gets no warning of its approach, and shock waves form where the flow abruptly slows or changes direction.

Context Anchor

Seen in discussions of transonic airflow, especially around wings and control surfaces at high speeds.

Derivation

From Latin 'super' meaning 'above' and 'sonic' from 'sonus' meaning 'sound.' So 'supersonic' literally means 'above sound ' — the air is moving faster than through it.