Plain English Meaning

Slower than the speed of sound. The airplane, or the air moving over it, is moving slower than sound travels through that same air.

Full Definition

Describing speeds below the speed of sound. In aerodynamics, airflow is considered subsonic when it moves at less than Mach 1 (the local speed of sound), so air behaves as a smooth, compressible-but-not-shock-forming flow over the airframe.

Context Anchor

Seen in turboprop and propeller discussions, especially when describing airflow and propeller blade tip speed.

Derivation

From the Latin 'sub-' meaning 'below' and 'sonus' meaning 'sound.' Literally 'below sound' — below the speed of sound.

Why Pilots Care

Helps pilots recognize safe operating ranges where drag remains predictable and the behaves as expected.