In C programming, a constant is a value that cannot be changed during program execution. Constants are declared using the const keyword and can be of various types, including integer, floating-point, and character.

Here is an example of declaring and using a constant:

#include <stdio.h>

int main() {
  const int num = 10; // declaring and initializing a constant variable
  
  printf("The value of num is %d\n", num); // accessing the value of the constant variable
  
  // attempting to modify the value of the constant variable will result in a compiler error
  // num = 20; // error: assignment of read-only variable 'num'
  
  return 0;
}

In this example, we declared a constant variable named num inside the main() function. The const keyword tells the compiler that the value of the variable cannot be changed during program execution. The variable is initialized to 10 when the program starts, and its value cannot be modified from within the program.

Constants are often used to represent fixed values in a program, such as mathematical constants or program configuration settings. They can also be used to make code more readable and maintainable, by giving meaningful names to values that would otherwise be hard-coded.

It is important to note that constants cannot be modified once they are declared, and attempting to modify the value of a constant variable will result in a compiler error. Constants should be used for values that are known at compile-time and are not expected to change during program execution.

In summary, a constant in C programming is a value that cannot be changed during program execution. Constants are declared using the const keyword and are often used to represent fixed values in a program. They cannot be modified once they are declared and should be used for values that are known at compile-time and are not expected to change during program execution.