Define the C Preprocessor?

Preprocessor' is a translation stage that is applied to your source code before the compiler proper gets its hands on it usually the preprocessor performs textual substitutions on your source code.

The pre-processor basically processes text; it knows virtually nothing about C syntax. It scans the input program text look for the directives in which the first character on a line is a '#'. While it convenes these directives, it takes certain actions including other files to be processed defining symbols, and so on it acts completely on the program text and will happily pre-process text which may be complete gibberish for the C compiler. This pre-processed text is after that passed on to the C compiler itself which checks the C syntax and if it consumes it, likes it, translating it into some intermediate code which is output, ready for input to the next phase.