The golden ratio is something very commonly used in math, art, architecture, geometry, etc.

It is represented by the Greek number phi (φ or ϕ) which is equal to 1.6180339887.., that goes on infinitely, making it an irrational number.

The golden ratio is when the ratio of 2 quantities is the same as the ratio of the larger quantity to the sum of the 2. (a+b/a = a/b).

The golden ratio is related very closely to the Fibonacci sequence; when 2 consecutive numbers from the sequence are taken, their ratio is very close to that of the golden ratio.

It’s unknown who exactly discovered the golden mean/the number phi, since it’s been said to have existed forever.  Although, ancient Greeks Phidias, Plato, and Euclid have played a great role in the study of the golden ratio.