An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic pump with no moving parts, excepting a valve to control inlet flow.
A steam injector is a typical application of the principle used to deliver cold water to a boiler against its own pressure, using its own live or exhaust steam, replacing any mechanical pump. When first developed, its operation was intriguing because it seemed paradoxical, almost like perpetual motion, but it was later explained using thermodynamics. Other types of injector may use other pressurised motive fluids such as air.
Depending on the application, an injector can also take the form of an eductor-jet pump, a water eductor or an aspirator. An ejector operates on similar principles to create a vacuum feed connection for braking systems etc.