Nozzle

A nozzle is a pipe or tube of varying diameter that is used to direct or modify the flow of a liquid or gas. Nozzles are frequently used to control the rate of flow, and/or the pressure of the stream that emerges from them.

Nozzles can be described as convergent (narrowing down from a wide diameter to a smaller diameter in the direction of the flow) or divergent (expanding from a smaller diameter to a larger one). Some nozzles are combinations of both - for example, rocket engines use a nozzle that is usually described as a CD nozzle (for convergent-divergent) first narrowing to a "throat" then expanding again into a "bell".