Venturi

A short constriction in a tube between two longer tapered portions that are usually of unequal length but terminate with the same diameter, so that there is a drop in pressure in a fluid flowing through the constriction which may be used to determine the rate of flow or used as a source of suction; also devices having this form and the effect involved." (OED.) Another way of putting it, is, that the speed of a fluid flowing through a tube can be accelerated by introducing a tapering constriction into the flow path. Bernoulli’s principle tells us that Venturi’s constriction will also lower the fluid pressure, since an increase in velocity must lead to a decrease in pressure.

See also: Bernoulli′s Principle, Venturi Giovanni.