What does undefined symbols for architecture x86_64 mean in C?-Collection of common programming errors

This is my code.

#include 

int main(){
int first;
int second;
int third;
float average=0.0;

    printf ("This program will find the average of 3 numbers.\n");
    delay(1000);
    printf ("Type the first number.\n");
    scanf ("%d", &first);
    printf ("Type the second number.\n");
    scanf ("%d", &second);
    printf ("Type the third number.\n");
    scanf ("%d", &third);
    average = (first+second+third)/2.0;
    printf ("The average of %d, %d, and %d is %.3f\n", first, second, third, average);

return (0);
}

When I use gcc to compile it says

Undefined symbols for architecture x86_64:
  "_delay", referenced from:
      _main in cc62F0GD.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

I am a beginner in coding. What does this mean and how do I fix it?

Originally posted 2013-11-27 11:51:48.