Definition
The ambient atmospheric pressure of the air surrounding the airplane, sensed through a static port (a small flush opening on the fuselage or pitot-static probe) and supplied to the altimeter, vertical speed indicator, and airspeed indicator. It represents the pressure of still air that is not affected by the airplane's forward motion.
Plain English
It is the pressure of the calm air around the airplane, measured through a small hole in the side of the aircraft, and used by several flight instruments to show altitude, climb rate, and airspeed.
Context Anchor
You will see this term in discussions of pitot-static instruments, takeoff in ground effect, and situations where airflow around the airplane can make instrument readings slightly inaccurate.
Derivation
Static comes from the Latin staticus, meaning 'standing still.' The static source senses the pressure of air that is standing still relative to the atmosphere, as opposed to the moving air rammed into the pitot tube.
Why Pilots Care
It supplies the reference pressure for the altimeter, vertical-speed indicator, and airspeed indicator; small errors directly affect altitude and speed indications.
Grounding Statement
The airplane is trying to sample the normal pressure of the outside air, not the extra pressure made by air hitting the airplane as it moves.
Intuition Check
“Static” does not mean electrical static here, and it does not mean the airplane is not moving. It means the pressure of the surrounding air, separate from the pressure created by forward motion.
Example Sentence 1
In ground effect, changes in airflow near the static source pressure port can cause the airspeed indicator to read slightly low.
Example Sentence 2
A blocked static source caused the altimeter to freeze even though the airplane continued to climb.