Golden ratio
With a number namely a and another smaller number b, the ratio of the two numbers is found by dividing them. Their ratio is a/b. Another ratio is found by adding the two numbers together  and dividing this by the larger number a.  The new ratio is (a+b)/a. If these two ratios are equal to the same number, then that number is called the golden ratio.  The Greek letter  (phi) is usually used as the name for the golden ratio.