A function call made by a recursive function to another instance of itself. For example, recursively finding the length of a linked list (in C):

int bleah(LinkedList * list, int * snarf)
{
  if(list) {
    bleah(list->next, snarf);  /* the recursive call */
  }
  else {
    /* this is the base case, don't recurse further */
  }
  return ++(*snarf);
}

int main()
{
  int snarf = -1;
  return bleah(huge_global_list, &snarf);
}