A nozzle is a mechanical device designed to control the characteristics of a fluid flow as it exits from an enclosed chamber into some medium. Frequently the goal is to increase the kinetic energy of the flowing medium at the expense of its pressure energy . A nozzle is generally a pipe or tube of varying diameter, and it can be 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. The shape of the nozzle determines how efficiently the expansion of the exhaust gases is converted into linear motion; in particular this determines the exhaust velocity and therefore the specific impulse. For example, the optimal shape of a rocket nozzle depends on the ambient pressure; for rockets travelling from the Earth to orbit, a simple nozzle design is only optimal at one altitude, losing efficiency and wasting fuel at other altitudes. A number of sophisticated designs have been proposed, such as the expanding nozzle, the aerospike, and the plug nozzle which adapt in some way to changing ambient pressure.

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.