Plain English Meaning
The small enclosed space inside the airspeed indicator that receives the air pressure piped in from the pitot tube. As that pressure rises with speed, it pushes on a flexible diaphragm inside the chamber, which moves the needle on the gauge.
Full Definition
The sealed internal cavity inside the airspeed indicator into which ram (pitot) air pressure is fed through a line from the pitot tube. The pressure inside this chamber acts on a diaphragm, which expands or contracts as the ram pressure changes. The diaphragm's movement is mechanically linked to the airspeed needle.
Context Anchor
Seen in pitot-static system diagrams, especially when learning how the airspeed indicator uses pitot pressure and .