What is a carburetor venturi?

A venturi is also called a "jet pump", an "injector", or an "eductor". As a fluid velocity is increased, its pressure decreases. A venturi is sued to increase fluid velocity, to obtain the lower pressures necessary to draw in some other fluid. Commonly used to drain water beds using flow from a sink, to add chemicals to a water stream, and to add gasoline to an air stream (in older engines).