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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, direction, 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 and supersonic jet 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".

The design of an efficient nozzle can drastically affect the efficiency of a rocket engine, and so a number of sophisticated designs have been proposed, such as the expanding nozzle, the aerospike, and the plug nozzle.

Magnetic nozzles have also been proposed for some types of propulsion, in which the flow of plasma is directed by magnetic fields instead of walls made of solid matter.