phi

Etymology

The OED takes it as borrowed from Late Latin phi, recorded from the late 4th century,^[2] from Ancient Greek φεῖ (pheî), the Greek name of the letter Φ, φ.^[2][3] The American Heritage Dictionary gives the later form φῖ (phî) as the immediate source,^[4] and Merriam-Webster derives the word from Middle Greek.^[5]

Earliest recorded meaning

The name of the twenty-first letter of the Greek alphabet: perhaps before 1425, in Mandeville's Travels.^[2][6] Merriam-Webster also dates it to the 15th century.^[5]

Semantic evolution

The letter was originally an aspirated p, as in upheave, and later came to be pronounced f;^[3] the Romans wrote it ph, which is why English spells Greek words that way.^[7][3] As a Greek numeral, φ with a stroke stood for 500.^[3] The Shorter Oxford notes that the English word was rare before the mid 19th century.^[6] In the 20th century the letter became the name of technical quantities: a measure of the size of sediment particles in geology (1934),^[2][6] the phi coefficient in statistics,^[6] and the phi meson in particle physics (1962).^[2][6] In mathematics φ often denotes a plane angle or a polar coordinate.^[8]

Timeline

• c. 1425 – the name of the Greek letter Φ, φ, in Mandeville's Travels.^[2][5]
• 1934 – in geology, an index of particle size.^[2][6]
• 1962 – in particle physics, the phi meson.^[2][6]

Meanings

Noun

1. The twenty-first letter of the Greek alphabet (Φ, φ), transliterated as ph or f.^[9][5][4][2]
2. (geology) An index of the size of sediment particles, the negative logarithm to base 2 of the diameter in millimetres.^[2][6]
3. (particle physics) A meson of a group with the same quantum numbers as the omega meson; in full, phi meson.^[2][6]
4. (statistics) In phi coefficient, a measure of correlation between two variables that can each take only two values.^[6]

Word family

• phi phenomenon

Related words

• ph

References

1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d The Chambers Dictionary (2014), 13th ed., Edinburgh: Chambers
4. ^ a b The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
5. ^ a b c d Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
6. ^ a b c d e f g h i j Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
7. ^ Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
8. ^ Soanes, Catherine; Stevenson, Angus, eds. (2004), Concise Oxford English Dictionary, 11th ed., Oxford: Oxford University Press
9. ^ Collins English Dictionary (2023), Glasgow: HarperCollins