Epsilon

The letter Ε was taken over from the Phoenician letter He () when Greeks first adopted alphabetic writing. In archaic Greek writing, its shape is often still identical to that of the Phoenician letter. Like other Greek letters, it could face either leftward or rightward (), depending on the current writing direction, but, just as in Phoenician, the horizontal bars always faced in the direction of writing. Archaic writing often preserves the Phoenician form with a vertical stem extending slightly below the lowest horizontal bar. In the classical era, through the influence of more cursive writing styles, the shape was simplified to the current E glyph.[6]

While the original pronunciation of the Phoenician letter He was [h], the earliest Greek sound value of Ε was determined by the vowel occurring in the Phoenician letter name, which made it a natural choice for being reinterpreted from a consonant symbol to a vowel symbol denoting an [e] sound.[7] Besides its classical Greek sound value, the short /e/ phoneme, it could initially also be used for other [e]-like sounds. For instance, in early Attic before c. 500 BC, it was used also both for the long, open /ɛː/, and for the long close /eː/. In the former role, it was later replaced in the classic Greek alphabet by Eta (Η), which was taken over from eastern Ionic alphabets, while in the latter role it was replaced by the digraph spelling ΕΙ.

Some dialects used yet other ways of distinguishing between various e-like sounds.

In Corinth, the normal function of Ε to denote /e/ and /ɛː/ was taken by a glyph resembling a pointed B (), while Ε was used only for long close /eː/.[8] The letter Beta, in turn, took the deviant shape .

In Sicyon, a variant glyph resembling an X () was used in the same function as Corinthian .[9]

In Thespiai (Boeotia), a special letter form consisting of a vertical stem with a single rightward-pointing horizontal bar () was used for what was probably a raised variant of /e/ in pre-vocalic environments.[10][11] This tack glyph was used elsewhere also as a form of "Heta", i.e. for the sound /h/.

After the establishment of the canonical classical Ionian (Eucleidean) Greek alphabet, new glyph variants for Ε were introduced through handwriting. In the uncial script (used for literary papyrus manuscripts in late antiquity and then in early medieval vellum codices), the "lunate" shape () became predominant. In cursive handwriting, a large number of shorthand glyphs came to be used, where the cross-bar and the curved stroke were linked in various ways.[12] Some of them resembled a modern lowercase Latin "e", some a "6" with a connecting stroke to the next letter starting from the middle, and some a combination of two small "c"-like curves. Several of these shapes were later taken over into minuscule book hand. Of the various minuscule letter shapes, the inverted-3 form became the basis for lower-case Epsilon in Greek typography during the modern era.

Uncial	Uncial variants	Cursive variants	Minuscule	Minuscule with ligatures

Despite its pronunciation as mid, in the International Phonetic Alphabet, the Latin epsilon /ɛ/ represents open-mid front unrounded vowel, as in the English word pet /pɛt/.

The uppercase Epsilon is not commonly used outside of the Greek language because of its similarity to the Latin letter E. However, it is commonly used in structural mechanics with Young's Modulus equations for calculating tensile, compressive and areal strain.

The Greek lowercase epsilon ε, the lunate epsilon symbol ϵ, or the Latin lowercase epsilon ɛ (see above) is used in a variety of places:

• In engineering mechanics, strain calculations ϵ = increase of length / original length. Usually this relates to extensometer testing of metallic materials.
• In mathematics
  • (particularly calculus), an arbitrarily small positive quantity is commonly denoted ε; see (ε, δ)-definition of limit.
    • In reference to this, the late mathematician Paul Erdős also used the term "epsilons" to refer to children (Hoffman 1998, p. 4).
  • Hilbert introduced epsilon terms ${\displaystyle \epsilon x.\phi }$ as an extension to first order logic; see epsilon calculus.
  • it is used to represent the Levi-Civita symbol.
  • it is used to represent dual numbers: a + bε, with ε² = 0 and ε ≠ 0.
  • it is sometimes used to denote the Heaviside step function.[13]
  • in set theory, the epsilon numbers are ordinal numbers that satisfy the fixed point ε = ω^ε. The first epsilon number, ε₀, is the limit ordinal of the set {ω, ω^ω, ω^ωω, ...}.
• In computer science, it often represents the empty string, though different writers use a variety of other symbols for the empty string as well; usually the lower-case Greek letter lambda (λ).
• In computer science, the machine epsilon indicates the upper bound on the relative error due to rounding in floating point arithmetic.
• In physics,
  • it indicates the permittivity of a medium; with the subscript 0 (ε₀) it is the permittivity of free space.
  • it can also indicate the strain of a material (a ratio of extensions).
• In automata theory, it shows a transition that involves no shifting of an input symbol.
• In astronomy,
  • it stands for the fifth-brightest star in a constellation (see Bayer designation).
  • Epsilon is the name for the most distant and most visible ring of Uranus.
  • In planetary science, ε denotes the axial tilt.
• In chemistry, it represents the molar extinction coefficient of a chromophore.
• In economics, ε refers to elasticity.
• In statistics,
  • it is used to refer to error terms.
  • it also can to refer to the degree of sphericity in repeated measures ANOVAs.
• In agronomy, it is used to represent the "photosynthetic efficiency" of a particular plant or crop.