Preprocessing (in C/C++ language). The mechanism which reviews the input ".c/.cpp" file, executes the preprocessor's directives in it, includes the content of other files listed in #include directives into it etc. As the result you get a file which does not contain the preprocessor's directives, all the used macros are opened and instead of #include directives the content of the corresponding files is substituted. The file with the result of preprocessing usually has ".i" suffix. The result of preprocessing is called a translation unit.