Venturi effect

The upstream static pressure (1) is higher than in the constriction (2), and the fluid speed at "1" is lower than at "2", because the cross-sectional area at "1" is greater than at "2".
A flow of air through a Pitot tube Venturi meter, showing the columns connected in a manometer and partially filled with water. The meter is "read" as a differential pressure head in cm or inches of water.
Video of a Venturi meter used in a lab experiment
Idealized flow in a Venturi tube

The Venturi effect is the reduction in fluid pressure that results when a moving fluid speeds up as it flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th-century Italian physicist Giovanni Battista Venturi.

The effect has various engineering applications, as the reduction in pressure inside the constriction can be used both for measuring the fluid flow and for moving other fluids (e.g. in a vacuum ejector).