Definition
A speed less than the speed of sound. In aviation, subsonic speed refers to airflow over the entire aircraft remaining below Mach 1, meaning no part of the airflow has reached or exceeded the local speed of sound.
Plain English
Moving slower than sound travels through the air. The aircraft and the air flowing around it are all moving slower than the speed of sound.
Context Anchor
Seen in aerodynamics, aircraft performance, and discussions of high-speed flight.
Derivation
From the Latin 'sub' meaning 'below' or 'under,' combined with 'sonic' from the Latin 'sonus' meaning 'sound.' So 'subsonic' literally means 'below sound' — slower than sound travels.
Why Pilots Care
Indicates the absence of shock waves and compressibility effects that affect handling and drag.
Intuition Check
Subsonic does not mean slow. It only means slower than the speed of sound; a jet can be very fast and still be at subsonic speed.
Example Sentence 1
Light training aircraft like the Cessna 172 always operate at subsonic speed, well below Mach 1.
Example Sentence 2
Transition training begins once the student understands subsonic speed limits.