The Collaborative International Dictionary of English v.0.48:

Injection \In*jec"tion\, n. [L. injectio : cf. F. injection.]
   1. The act of injecting or throwing in; -- applied
      particularly to the forcible insertion of a liquid or gas,
      by means of a syringe, pump, etc.
      [1913 Webster]

   2. That which is injected; especially, a liquid inserted
      thrown into a cavity of the body by a syringe or pipe; a
      clyster; an enema. --Mayne.
      [1913 Webster]

   3. (Anat.)
      (a) The act or process of filling vessels, cavities, or
          tissues with a fluid or other substance.
      (b) A specimen prepared by injection.
          [1913 Webster]

   4. (Steam Eng.)
      (a) The act of throwing cold water into a condenser to
          produce a vacuum.
      (b) The cold water thrown into a condenser.
          [1913 Webster]

   Injection cock, or Injection valve (Steam Eng.), the cock
      or valve through which cold water is admitted into a
      condenser.

   Injection condenser. See under Condenser.

   Injection pipe, the pipe through which cold water is
      through into the condenser of a steam engine.

   fuel injection, a method of inserting fuel into
      internal-combustion engines by directly forcing the liquid
      fuel into the combustion chamber at an appropriate point
      in the piston cycle; in contrast to carburetion, in
      which an air-fuel mixture is drawn in by the downward
      stroke of the piston.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

injection
    n 1: the forceful insertion of a substance under pressure
    2: any solution that is injected (as into the skin) [syn:
       injection, injectant]
    3: the act of putting a liquid into the body by means of a
       syringe; "the nurse gave him a flu shot" [syn: injection,
       shot]

The Free On-line Dictionary of Computing (19 January 2023):

injection

   1.  A function, f : A -> B, is injective or
   one-one, or is an injection, if and only if

    for all a, b in A, f(a) = f(b) => a = b.

   I.e. no two different inputs give the same output (contrast
   many-to-one).  This is sometimes called an embedding.  Only
   injective functions have left inverses f' where f'(f(x)) = x,
   since if f were not an injection, there would be elements of B
   for which the value of f' was not unique.  If an injective
   function is also a surjection then is it a bijection.

   2.  An injection function is one which takes
   objects of type T and returns objects of type C(T) where C is
   some type constructor.  An example is

    f x = (x, 0).

   The opposite of an injection function is a projection
   function which extracts a component of a constructed object,
   e.g.

    fst (x,y) = x.

   We say that f injects its argument into the data type and fst
   projects it out.

   (1995-03-14)