In C programming, a function definition is a block of code that contains the instructions for performing a specific task. A function definition typically consists of four parts: the function header, the function body, any local variable declarations, and the return statement (if the function returns a value).

The syntax for a function definition in C is as follows:

return_type function_name(parameter_list)
{
    // Local variable declarations
    // Function body
    return value;
}

In this syntax, return_type is the data type of the value returned by the function (or void if the function does not return a value), function_name is the name of the function, and parameter_list is a comma-separated list of parameter declarations.

Here's an example of a function definition for a function that calculates the factorial of a number:

int factorial(int n)
{
    int result = 1;
    for (int i = 1; i <= n; i++) {
        result *= i;
    }
    return result;
}

In this example, the function is named factorial, it takes an integer argument n, and it returns an integer value that is the factorial of n. The function body contains the code that performs the factorial calculation, which is a loop that multiplies result by the value of i for each iteration. The final result is returned by the return statement.

Once a function is defined, you can call it from other parts of your program by using its name and passing in any required arguments. Functions can be called multiple times from different parts of your program, making them a powerful tool for breaking down complex tasks into smaller, more manageable functions.