Definition
The branch of aerodynamics that studies airflow and the forces it produces on an aircraft when the air moving over every part of the airframe stays below the speed of sound. In practice, this regime applies when the aircraft's flight speed is low enough that local airflow acceleration over wings and other surfaces does not reach Mach 1, generally at flight Mach numbers below about 0.75. In this regime, air behaves as an essentially incompressible fluid, and lift, drag, and pressure changes follow the classical principles taught in primary flight training.
Plain English
It's the study of how air flows around an airplane and creates lift and drag when the airplane is flying well below the speed of sound. At these speeds, the air can be treated as if it doesn't squeeze together, which keeps the behavior simple and predictable.
Context Anchor
Seen in high-speed flight discussions when comparing airflow below the speed of sound with airflow at or above the speed of sound.
Derivation
Sub-' comes from Latin meaning 'below,' and 'sonic' comes from Latin 'sonus' meaning 'sound.' So 'subsonic' literally means 'below the speed of sound,' which is exactly the airflow condition this branch of aerodynamics covers.
Why Pilots Care
Most training, general aviation, and commercial aircraft operate entirely in this regime, so understanding it explains lift, drag, and control characteristics they will use on every flight.
Grounding Statement
In subsonic flight, the air ahead of the aircraft can still be affected before the aircraft arrives there.
Intuition Check
Do not read subsonic as simply “slow.” In aviation, it means below the local speed of sound; a jet can be very fast and still be subsonic.
Example Sentence 1
Most general aviation flying takes place entirely within the subsonic aerodynamics regime, where standard lift and drag rules apply.
Example Sentence 2
Subsonic aerodynamics explains why the same airfoil produces reliable lift at typical training aircraft speeds.