Phi (uppercase Φ, lowercase or ; , pheî, ; modern , fi, ; English: ) is the 21st letter of the Greek alphabet. In Ancient Greek, it represented an aspirated voiceless bilabial plosive , which was the origin of its usual romanization as "ph". In modern Greek, it represents a voiceless labiodental fricative  and is correspondingly romanized as "f". Its origin is uncertain but it may be that phi originated as the letter qoppa and initially represented the sound before shifting to Classical Greek . In traditional Greek numerals, phi has a value of 500 or . The Cyrillic letter Ef (Ф, ф) descends from phi.

Phi (uppercase Φ, lowercase φ, or math symbol ϕ) is the 21st letter of the Greek alphabet.