Epsilon

Epsilon ( , uppercase Ε, lowercase ε or ϵ; Greek: έψιλον) is the fifth letter of the Greek alphabet, corresponding phonetically to a mid front unrounded vowel IPA: [e̞] or IPA: [ɛ̝]. In the system of Greek numerals it also has the value five. It was derived from the Phoenician letter He (𐤄‎). Letters that arose from epsilon include the Latin E, Ë and Ɛ, and Cyrillic Е, È, Ё, Є and Э. The name of the letter was originally εἶ (eî [êː]), but it was later changed to ἒ ψιλόν (è psilón 'simple e') in the Middle Ages to distinguish the letter from the digraph ⟨αι⟩, a former diphthong that had come to be pronounced [e], and because the digraph ⟨ει⟩ had become unsuitable due to its own shift to [i]. In Modern Greek, its name has fused into έψιλον (épsilon).

The uppercase form of epsilon is identical to Latin ⟨E⟩ but has its own code point in Unicode: U+0395 Ε GREEK CAPITAL LETTER EPSILON. The lowercase version has two typographical variants, both inherited from medieval Greek handwriting. One, the most common in modern typography and inherited from medieval minuscule, looks like a reversed number "3" and is encoded U+03B5 ε GREEK SMALL LETTER EPSILON. The other, also known as lunate or uncial epsilon and inherited from earlier uncial writing, looks like a semicircle crossed by a horizontal bar: it is encoded U+03F5 ϵ GREEK LUNATE EPSILON SYMBOL. While in normal typography these are just alternative font variants, they may have different meanings as mathematical symbols: computer systems therefore offer distinct encodings for them. In TeX, \epsilon (

ϵ

{\displaystyle \epsilon \!}

) denotes the lunate form, while \varepsilon (

ε

{\displaystyle \varepsilon }

) denotes the epsilon number. Unicode versions 2.0.0 and onwards use ɛ as the lowercase Greek epsilon letter, but in version 1.0.0, ϵ was used. The lunate or uncial epsilon provided inspiration for the euro sign, €.