Venturi

What is it?
A venturi is a device used to inject air, most often, or other gasses, into water flow.

A venturi is a narrowed part of a tube through which fluids (gas or liquid) are being pumped, which causes the pressure in the narrow section to exhibit lower pressure. Into this low pressure zone whatever desired gasses or liquids are introduced.

Links

 * Wikipedia article on the Venturi effect