High Quality Content by WIKIPEDIA articles! Phi, pronounced in modern Greek and or sometimes in English, is the 21st letter of the Greek alphabet. In modern Greek, it represents, a voiceless labiodental fricative. In Ancient Greek it represented [p?], an aspirated voiceless bilabial plosive (from which English ultimately inherits the spelling "ph" in words derived from Greek). In the system of Greek numerals it has a value of 500 (??) or 500,000 (??). The Cyrillic letter Ef (?, ?) arose from ?.