noun

• One who, or that which, injects something.
• One who injects (himself or herself with) psychoactive drugs.
• Any of various devices that are used to inject something, as:
• An apparatus by which a stream of water is forced into a steam boiler.
• Any of the one or more devices in a fuel injection system that create the spray of fuel.
• (object-oriented programming) An object that realizes a dependency injection.