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.
Plain English
Slower than the speed of sound. The airplane, or the air moving over it, is moving slower than sound travels through that same air.
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 aircraft behaves as expected.
Intuition Check
Subsonic does not simply mean "slow." It means below the local speed of sound, which changes with air conditions.
Example Sentence 1
General aviation airplanes operate entirely in the subsonic range, well below the speed of sound.
Example Sentence 2
Pilots monitor indicated airspeed to stay within subsonic limits during climb.