Definition
A sealed compartment within a pitot-static instrument that is connected to the aircraft's static port and holds outside ambient air pressure. The instrument's mechanism (such as an aneroid or diaphragm) reacts to changes in the pressure inside this chamber to drive the altimeter, vertical speed indicator, or airspeed indicator.
Plain English
It's the small enclosed space inside an instrument that holds the still, outside air pressure piped in from the static port. The instrument senses changes in that trapped pressure to show altitude, climb/descent rate, or airspeed.
Context Anchor
Seen in pitot-static instrument diagrams and discussions of how the altimeter, airspeed indicator, and vertical speed indicator receive static pressure.
Derivation
"Static" comes from the Greek statikos, meaning "causing to stand" or "at rest." In this context it refers to air that is not moving relative to the aircraft -- ambient pressure rather than ram (moving) air pressure. A "chamber" is simply an enclosed space. Together: an enclosed space holding still air pressure.
Why Pilots Care
Blockage or leaks in the static chamber produce incorrect altitude, vertical speed, and airspeed indications that can lead to loss of situational awareness.
Grounding Statement
Picture a small sealed space inside the instrument that is kept at the same pressure as the outside air around the airplane.
Intuition Check
Static does not mean electrical static here. It means air pressure used as a still, non-moving pressure reference.
Example Sentence 1
As the aircraft climbs, pressure inside the static chamber decreases, causing the altimeter to indicate a higher altitude.
Example Sentence 2
A blocked static port stops pressure from reaching the static chamber and causes the altimeter to freeze at its last reading.