Phi
/faɪ/noun
The twenty-first letter of the Greek alphabet, used as a symbol for the golden ratio, for angles, and for magnetic flux.
The Greek roots
Literally: “the Greek letter ph”
The story of the word
In classical Attic the letter was an aspirated p, not the f sound English gives it, which is why Roman scribes wrote it as ph rather than f. Its fame as the golden ratio is barely a century old: Mark Barr proposed φ in the early 1900s, choosing the initial of Phidias, who built the Parthenon sculptures. Euclid had described the same proportion without any symbol at all, calling it division in extreme and mean ratio.
- How it travelled
- Ancient Greek → Latin → English
In a sentence
He wrote phi on the board and drew the rectangle before anyone could ask what it meant.
The totient function is written with a phi, which trips up students who expect the golden ratio.