Plain English Meaning
A tube with a pinched middle. When air flows through it, the air speeds up where the tube narrows, and the pressure inside drops. That drop in pressure can be used to power some flight instruments.
Full Definition
A specially shaped tube that narrows in the middle and widens again at each end. As air flows through the narrow section (the throat), its speed increases and its pressure drops. In aircraft, a venturi tube mounted on the outside of the fuselage uses this pressure drop to drive air-powered gyroscopic instruments such as the attitude indicator and heading indicator on aircraft not equipped with an engine-driven vacuum pump.
Context Anchor
Pilots may encounter venturi tubes in discussions of older vacuum-driven flight instruments, carburetors, and airflow-measuring .