The Free On-line Dictionary of Computing (30 December 2018):

preprocessor
macro preprocessor

    A program that transforms input data in some way
   before it is read by the main program.  In the case of a
   compiler, the input is source code.  The main advantage of
   using a preprocessor is that it is possible to change the
   specification of the input data without changing the main
   program.  The separation can also help to make the system's
   overall behaviour easier to understand.  The disadvantage is
   that performance may be reduced by the extra input and output
   performed between the two programs.

   For example, the C preprocessor, cpp, handles textual
   macro substitution (it acts as a "macro preprocessor"),
   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.

   (2007-04-05)