Phi (uppercase Φ, lowercase φ or ϕfi] in modern Greekfaɪ] in English, is the 21st letter of the Greek alphabetf], a voiceless labiodental fricativepʰ], an aspirated voiceless bilabial plosive. In the system of Greek numerals it has a value of 500 (φʹ) or 500,000 (͵φ).
The lower-case letter φ (or often its variant, ) is used as a symbol for:
In HTML/XHTML, the upper and lower case phi character entity references are Φ (Φ in your browser) and φ (φ) respectively. In LaTeX, the math symbols are \Phi (Φ), \phi (φ), and \varphi ().