Plain English Meaning
A small gauge that shows whether the air running your spinning-gyro instruments (like the attitude indicator and heading indicator) is at the right strength. If the reading is wrong, those instruments may not be reliable.
Full Definition
An instrument that displays the pressure of the air being supplied to vacuum- or pressure-driven gyroscopic flight instruments, indicating whether that air is flowing through them at the correct value to keep their gyros spinning at the proper speed.
Context Anchor
Seen in discussions of vacuum or pressure systems that power air-driven attitude and heading instruments.