Definition
The static air pressure that remains sealed inside the aircraft's static system when the static port becomes blocked, holding the pressure at the value it was at the moment of blockage. Because the trapped pressure no longer changes with altitude, instruments referenced to the static system (altimeter, vertical speed indicator, and airspeed indicator) read incorrectly as the aircraft climbs or descends.
Plain English
It's the outside air pressure that gets stuck inside the static system when the static port plugs up. The pressure inside stays frozen at whatever it was when the blockage happened, even as the aircraft moves to a different altitude.
Context Anchor
You encounter this in instrument flying when learning what happens after a blocked static system and how it affects instrument indications.
Derivation
Trapped' here means held in place with no way out. Static pressure is the ambient air pressure acting on a surface that isn't moving through the air. Together: ambient pressure that has been sealed in and can no longer equalize with the outside.
Why Pilots Care
It causes the altimeter to freeze at the blockage altitude while the airspeed indicator and vertical speed indicator give false readings, creating a serious risk of altitude and speed misjudgment.
Analogy
It is like sealing a small bit of air inside a tube. The air inside still has the pressure it had when it was sealed, even if the air outside later changes.
Intuition Check
Do not read “static” as electrical static or radio noise here. It means still outside air pressure used by flight instruments. Do not read “trapped” as normal storage. Here it means a blockage is preventing the pressure from updating.
Example Sentence 1
After the static port iced over, the trapped static pressure caused the altimeter to hold steady at 6,000 feet even though the aircraft was descending.
Example Sentence 2
The pilot noticed the airspeed indicator behaving erratically on descent because of trapped static pressure in the system.