Plain English Meaning
Vacuum pressure is when the air pressure inside a part of the system is lower than the air pressure outside it. That difference makes outside air rush in, and that moving air is what spins the gyros in certain flight instruments.
Full Definition
A pressure lower than the surrounding atmospheric pressure, produced inside an instrument system by a vacuum pump. The pressure difference between outside air and the lower-pressure interior is what drives air through the system to spin the gyros in vacuum-powered flight instruments such as the attitude indicator and heading indicator.
Context Anchor
Seen when studying aircraft power sources for flight instruments, especially vacuum pumps, vacuum gauges, and vacuum-driven or .