Φ

Phi (uppercase Φ, lowercase φ or ϕ), pronounced [] in modern Greek and as [] in English, is the 21st letter of the Greek alphabet. In modern Greek, it represents [], a voiceless labiodental fricative. In Ancient Greek it represented [], an aspirated voiceless bilabial plosive. In the system of Greek numerals it has a value of 500 (φʹ) or 500,000 (͵φ).
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Porosity symbol