#define

The #define preprocessor directive is used to define constant or micro substitution. It can use any basic data type.
Syntax:
#define token value
Example: 
#include <stdio.h>   
#define PI 3.14  
main() {  
printf("%f",PI);  
}  
Output:
3.140000
see an example of #define to create a macro. 
#include <stdio.h>   
#define MIN(x,y) ((x)<(y)?(x):(y))  
void main() {  
printf("Minimum between 20 and 30 is: %d\n", MIN(20,30));    
  }  
Output:
Minimum between 20 and 20 is:  20

#undef

The #undef preprocessor directive is used to undefine the constant or macro defined by #define.
Syntax:

#undef token
see a simple example to define and undefine a constant. 
#include <stdio.h>   
#define PI 3.14  
#undef PI  
main() {  
printf("%f",PI);  
  }
Output:
Compile Time Error: 'PI' undeclared
The #undef directive is used to define the preprocessor constant to a limited scope so that you can declare constant again.
see an example where we are defining and undefining number variable. But before being undefined, it was used by square variable. 
#include <stdio.h>    
#define number 20  
int square=number*number;  
#undef number  
main() {  
printf("%d",square);  
 }  
Output:
400

#ifdef

The #ifdef preprocessor directive checks if macro is defined by #define. If yes, it executes the code otherwise #else code is executed, if present.
Syntax:

#ifdef MACRO //code #endif
Syntax with #else:
#ifdef MACRO //successful code #else //else code #endif

#ifdef example

see a simple example to use #ifdef preprocessor directive. 
#include <stdio.h>    
#define NOINPUT  
void main() {  
int b=0;  
#ifdef NOINPUT  
b=4;  
#else  
printf("Enter b:");  
scanf("%d", &b);  
#endif         
printf("Value of b: %d\n", b);  
getch();  
 }  
Output:
Value of b: 4
But, if you don't define NOINPUT, it will ask user to enter a number. 
#include <stdio.h>    
void main() {  
int b=0;  
#ifdef NOINPUT  
b=4;  
#else  
printf("Enter b:");  
scanf("%d", &b);  
#endif         
printf("Value of b: %d\n", b);  
  }  
Output:
Enter b:10
Value of b: 10