Bernoulli's principle says that as a fluid flows through a tube or pipe, when the pipe narrows the speed of the flow increases, but its pressure drops. A venturi is the name for the restricted, or narrowed, area of a container through which a fluid flows, and its purpose can be to take advantage of either the increase in fluid speed, or the drop in pressure, or both.
A typical application of a venturi is inside a carburetor. When air is being drawn into the cylinders of the engine, it passes through a venturi in which there is an inlet connected to a source of fuel vapor. The lower pressure of the air causes some of the higher pressure fuel vapor to be pushed into and mixed with the stream of air.
