the.com/golden ratio

means An irrational number, roughly 1.618, where the ratio of two quantities equals the ratio of their sum to the larger one — a proportion many artists and architects have treated as uniquely pleasing to the eye.

from The ratio itself was studied by ancient Greek mathematicians, notably Euclid, who called it simply 'extreme and mean ratio' — no mysticism attached yet. The reverential name 'golden ratio' (and the symbol φ, after the sculptor Phidias) is a much later, 19th-century habit, part of a wave of enthusiasm that retroactively found the proportion everywhere from the Parthenon to seashells. Many of those claimed sightings — including the Parthenon link — are now considered folk etymology of a sort: enthusiastic pattern-matching rather than evidence the ancient builders designed by it on purpose.