Source : Free On-Line Dictionary of Computing

preprocessor
     
        A program invoked by various {compiler}s to process code
        before compilation.  For example, the {C} preprocessor, {cpp},
        handles textual {macro} substitution, {conditional
        compilation} and inclusion of other files.  A preprocessor may
        be used to transform a program into a simpler language, e.g. to
        transform {C++} into {C}.
     
        (1994-11-09)