the.com/phi

means An irrational number, approximately 1.618, defined by the property that a line divided into two parts has the same ratio of the whole to the larger part as the larger part to the smaller — commonly called the golden ratio.

from The number itself was studied since antiquity under the name 'extreme and mean ratio,' but the symbol φ (phi) is a modern naming: a 20th-century American mathematician popularized using the Greek letter, often said to honor the sculptor Phidias, who allegedly employed the ratio in the Parthenon's proportions. That Phidias connection is largely folklore — there's no solid ancient evidence he deliberately used it — but the letter stuck as the symbol regardless, borrowed simply from the Greek alphabet like its cousins pi and tau.