The Collaborative International Dictionary of English v.0.48:

Injector \In*ject"or\, n.
   1. One who, or that which, injects.
      [1913 Webster]

   2. (Mach.) A contrivance for forcing feed water into a steam
      boiler by the direct action of the steam upon the water.
      The water is driven into the boiler by the impulse of a
      jet of the steam which becomes condensed as soon as it
      strikes the stream of cold water it impels; -- also called
      Giffard's injector, from the inventor.

   fuel injector, a device for actively injecting fuel into an
      internal-combustion engines by directly forcing the liquid
      fuel into the combustion chamber at an appropriate point
      in the piston cycle; -- an alternative to a carburetor,
      in which an air-fuel mixture is drawn in by the downward
      stroke of the piston.
      [1913 Webster]

WordNet (r) 3.0 (2006):

injector
    n 1: a contrivance for injecting (e.g., water into the boiler of
         a steam engine or particles into an accelerator etc.)