A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube that is pinched in the middle, making an hourglass-shape. It is used as a means of accelerating the flow of a gas passing through it. It is widely used in some types of steam turbine and is an essential part of the modern rocket engine.
The nozzle was developed by Swedish inventor Gustaf de Laval in the 19th century. Its operation relies on the different properties of gases flowing at subsonic and supersonic speeds. The speed of a subsonic flow of gas will increase if the pipe carrying it narrows. This is because a subsonic flow behaves (roughly) as if it is incompressible. At supersonic speeds, the reverse is true, and widening the pipe will increase the speed of the gas flowing through it.
A de Laval nozzle, then, functions by narrowing down to a "throat" at which point the gas flow is moving at the speed of sound, and then widening again to further accelerate the speed of the gas.
This principle was first used in a rocket engine by Robert Goddard. Walter Thiel's implementation of it made the V2 rocket possible.