Preprocessor directives are the special instructions which are executed before code passes through the compilation process.
It begins with hash (#) symbol and do not require semicolon at the end.
One of the most important preprocessor directive is #include which is used for including header file. It is also known as file inclusion. These files contains definition of pre-defined functions like printf(), scanf() etc.
For Example:
#include<stdio.h>
#Include<stdlib.h>
#include<string.h>
And #define is another commonly used preprocessor directive which is used for creating symbolic constants or for defining macros. It is also known as macro definition.
For Example:
#define g 9.81
#define MIN(a,b) a<b?a:b