Epsilon (uppercase Ε, lowercase ε; _el. Έψιλον) is the fifth letter of the Greek alphabet, corresponding phonetically to a close-mid front unrounded vowel /e/. In the system of Greek numerals it has a value of 5. It was derived from the Phoenician letter He .

"Epsilon" (polytonic|ἒ ψιλόν, "simple e") was coined in the Middle Ages to distinguish the letter from the diphthong αι, which started being pronounced the same way during the period of New Testament Greek.

The standard symbol for lowercase epsilon is the lunate epsilon ϵ (epsilon, epsilon,, in LaTeX), which has its origins in Medieval Greek.

In mathematical notation, the minuscule open e symbol ɛ (varepsilon, varepsilon, in LaTeX) from the extended Latin alphabet is often used interchangeably with the lunate epsilon.

The lunate epsilon ϵ is not to be confused with the set symbol in or falsely recognized as the lunate version Σ (Sigma).

Upper case: character code 0395Lower case: character code 03B5


The upper-case Epsilon is not a commonly-used symbol outside of the Greek language because of its similarity to the Roman letter E.

The lower-case epsilon, ε/ϵ, or open e, ɛ, (see above) is used as the symbol for:
* In mathematics (particularly calculus), an arbitrarily (or nearly so) small positive quantity is commonly denoted ε; see limit.
** By analogy with this, the late mathematician Paul Erdős also used the term "epsilons" to refer to children (Hoffman 1998, p. 4).
* In mathematics, the Levi-Civita symbol.
* In mathematics, to represent the dual numbers: a + bε, with ε2=0 and ε≠0.
* In mathematics, sometimes used to denote the Heaviside step function
* In set theory, the limit ordinal of the sequence omega,omega^{omega},omega^{omega^{omega,dots.
* In computing, the precision of a numeric data type and floating-point machine epsilon.
* In computer science, the empty string, though different writers use a variety of other symbols for the empty string as well, including the lower case Greek letter lambda.
* In physics, the permittivity of a medium.
* In physics/electronics, the EMF of a circuit
* In physics, the strain of a material (A ratio of extensions).
* In automata theory, a transition that involves no shifting of an input symbol.
* In astronomy, the fifth brightest (usually) star in a constellation. "See" Bayer designation.
* In the International Phonetic Alphabet, the open-mid front unrounded vowel, (but generally written ɛ instead of ε) as in the English word "pet" (/pɛt/)
* In astronomy, Epsilon is the name for Uranus' most distant and most visible ring.
* In chemistry, the molar extinction coefficient of a chromophore.
* In economics, ε refers to elasticity.

Fictional names

* In the popular web series "Red vs. blue", Epsilon is the name of the AI assigned to freelancer Washington.
* "Epsilon in Malaysian Pale" is the name of the second solo album released by Tangerine Dream leader Edgar Froese in 1975.
* Epsilon is also an Australian light-powered female-looking robot that fought Pluto in an episode of Astro Boy.
* "Epsilon Eridani III" is the planet that the spacestation Babylon 5 orbited in the eponymous sci-fi television series.
* E-104 Epsilon is also the name of a robot in Sonic Adventure.
* "Epsilon 9" is the name of the Federation space station consumed by the V'ger cloud in .
* Epsilon is the name of an unlockable creature in the game Monster Rancher 2
* In Greek it is used for Epsilon Team.
* "Epsilon Mirror" is the name of one of Aika's moves in the video game Skies of Arcadia.
* "Epsilon" is the leader of the Rebellion in
* "The Epsilon Program" is a mysterious cult in the video game by Game Developer Rockstar North. This cult is thought to be based on Scientology.
* The Epsilon Project is a fictional experiment mentioned in the Dreamcast and Xbox versions of Dead or Alive 2 and is based around cloning.
* Epsilon is the lowest of five social castes in the Aldous Huxley novel "Brave New World".
* "Plan Epsilon" was one of many plans to restore Doctor Doom to life in the event of his death, as shown in Fantastic Four issue no. 246, written by Stan Lee.
* Epsilon-Eagle is the name of the protagonist in the Sega Mega Drive game Alien Soldier.
* Epsilon was the name given to one of the ground support teams in the Sony Playstation game G-Police
* Epsilon is the name of the newly resurrected Yuri Faction in the Yuri's Revenge mod Mental Omega

Other uses

*"Epsilon" is the name used by General Motors for the post-2003 Epsilon platform.
* EpsilonEuskadi carmaker [ [http://www.epsiloneuskadi.com/ EPSILON EUSKADI=JOAN VILLADELPRAT & SERGIO RINLAND & JORDI CATON=AZKOITIA (43º 10' 22' N - 2º 19' 26' O) ] ] and EpsilonEuskadi LMP1 car.
*Hurricane Epsilon, a storm of the 2005 Atlantic hurricane season.
*"Epsilon Concepts" is the name of a national digital agency.


*Hoffman, Paul; "The man who loved only numbers". Hyperion, 1998. ISBN 0-7868-6362-5.


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