- Phi (letter)
**Phi**(uppercase**Φ**, lowercase**φ**or**Unicode|ϕ**), pronounced [IPA|fī] in modern Greek and as [IPA|faɪ] in English, is the 21st letter of theGreek alphabet . In modern Greek, it represents [IPA|f] , avoiceless labiodental fricative . In Ancient Greek it represented [IPA|pʰ] , an aspiratedvoiceless bilabial plosive (from which English ultimately inherits the spelling "ph" in words derived from Greek). In the system ofGreek numerals it has a value of 500 (φʹ) or 500,000 (͵φ).The lower-case letter $varphi\; ,$ (or often its variant, $phi\; ,$) is used as a symbol for:

*Thegolden ratio 1.618... in mathematics, art, and architecture.

*Euler's totient function φ(n) in number theory; also called "Euler's phi function".

*Theprobability density function of thenormal distribution inmathematics andstatistics .

*Anangle , typically the second angle mentioned, after "θ" (theta ). Especially:

**The argument of acomplex number .

**The phase of a wave in signal processing.

**Inspherical coordinates , mathematicians usually refer to phi as the polar angle (from the "z"-axis). The convention in physics is to use phi as the azimuthal angle (from the "x"-axis).

**One of thedihedral angle s in the backbones ofprotein s.

**Internal or effectiveangle of friction

*Electric potential in physics.

*Thework function in electronics.

*Ashorthand representation for anaromatic functional group inorganic chemistry

*Thefugacity coefficient inthermodynamics

*The ratio of free energy destabilizations of proteinmutant s inphi value analysis

*Incartography andnavigation ,latitude .

*A sentence infirst-order logic .

*Porosity in geology.The upper-case letter Φ is used as a symbol for:

*The golden ratio conjugate 0.618... in mathematics.

*Themagnetic flux andelectric flux in physics, with subscripts distinguishing the two.

*Thecumulative distribution function of thenormal distribution inmathematics andstatistics .

*Philosophy .

*Strength (or resistance) reduction factor instructural engineering , used to account for statistical variabilities in materials and construction methods.

*The number of phases in a power system inelectrical engineering , for example 1Φ forsingle phase , 3Φ forthree phase .

*The symbol of thevoiceless bilabial fricative in theinternational phonetic alphabet .Thediameter symbol inengineering ,⌀ , is often incorrectly referred to as "phi". This symbol is used to indicate the diameter of a circular section, for example ⌀14 means the diameter of thecircle is 14 units.**Computing**In

Unicode , there are multiple forms of the phi letter:

* upper case:

** U+03A6 GREEK CAPITAL LETTER PHI (Φ): Greek capital letter phi

* lower case:

** U+|03C6 GREEK SMALL LETTER PHI (φ): "letter" phi, used in Greek texts.

** U+03D5 GREEK PHI SYMBOL (Unicode|ϕ): phi "symbol", for mathematical and technical contexts. [*[*]*http://unicode.org/reports/tr25/#_Toc231 UTR #25: Unicode and Mathematics — Representative Glyphs for Greek Phi*]In HTML/

XHTML , the upper and lower case phicharacter entity reference s are`&Phi;`(Φ) and`&phi;`(φ) respectively.In

LaTeX , the math symbols are`Phi`($Phi,!$),`phi`($phi\; ,!$), and`varphi`($varphi,!$).In some browsers (e.g.

Internet Explorer 6 ), the shapes of the U+03C6 GREEK SMALL LETTER PHI (which should be viewed as a curl) and U+03D5 GREEK PHI SYMBOL (which should be viewed as a circle crossed by a slash) are exchanged. Compare these samples to check your browser::Phi letter: right $varphi,!$; possible wrong Unicode|φ

:Phi symbol: right $phi,!$; possible wrong Unicode|ϕ**ee also***F, f - Ef (Latin)

*Ф, ф - Ef (Cyrillic)**References**

*Wikimedia Foundation.
2010.*

### Look at other dictionaries:

**Phi Iota Alpha**— ΦIΑ Founded December 26, 1931 (1931 12 26) (79 … Wikipedia**Phi Kappa Psi**— ΦΚΨ Founded February 19, 1852 (1 … Wikipedia**Phi Delta Theta**— (ΦΔΘ) is an international fraternity founded in 1848 and headquartered at Miami University in Oxford, Ohio. Phi Delta Theta, Beta Theta Pi, and Sigma Chi form the Miami Triad. The fraternity has about 160 active chapters and colonies in over 43 U … Wikipedia**Phi Mu Alpha Sinfonia**— ΦΜΑ Founded October 6, 1898 (1898 10 06) (113 years ago) New England Conservatory of Music … Wikipedia**Phi**— could refer to:*Phi, the Greek letter Φ,φ *The Golden ratio *Euler s totient function *Phi phenomenon *A statistical measure of association reported with the chi square test *Phi, Thai demon spirits, such as Phi Song Nang (succubi) *A function in … Wikipedia**Phi Kappa Psi**— (ΦΚΨ, Phi Psi) est une Fraternité étudiante américaine. Sommaire 1 Histoire 2 Profession de foi 3 Symboles 4 L ordre du S.C … Wikipédia en Français**Phi Lambda Chi**— (ΦΛX) is a U.S. national fraternity founded in 1925. HistoryThe Lamb Society was organized in 1920 as an organization for high school boys. Each year this society was composed of boys who were interested in conditions which would help to keep up… … Wikipedia**Phi Sigma**— is an honor society for students of biological sciences. Phi Sigma was founded on March 17, 1915 at Ohio State University. It became a member of the Association of College Honor Societies in 1950. The Greek letter, Phi and Sigma, signify fellows… … Wikipedia**PHI**— is a three letter acronym or abbreviation that can refer to:* Post Polio Health International * Protected Health Information as part of the HIPAA regulations * The Golden ratio * The Philippines * Philadelphia, Pennsylvania ** Philadelphia… … Wikipedia**phi**— [fī, fē] n. [MGr phi < Gr phei] the twenty first letter of the Greek alphabet (Φ, φ) … English World dictionary