Phi (uppercase Φ, lowercase φ or math symbol ϕ), pronounced /ˈfaɪ/ or sometimes /ˈfiː/ in English, and /ˈfi/ in modern Greek, is the 21st letter of the Greek alphabet. In modern Greek, it represents [f f], a voiceless labiodental fricative. In Ancient Greek it represented [pʰ pʰ], an aspirated voiceless bilabial plosive (from which English ultimately inherits the spelling "ph" in words derived from Greek).
