Epsilon (uppercase Ε, lowercase ε) 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 . Letters that arose from Epsilon include the Roman E and Cyrillic Ye.
The standard symbol for lowercase epsilon is the lunate epsilon ϵ (ε in MathML), which has its origins in Medieval Greek.
In mathematical notation, the minuscule open e symbol ɛ ( in MathML) from the extended Latin alphabet is often used interchangeably with the lunate epsilon.
The lower-case epsilon, ε/ϵ, or open e, ɛ, (see above) is used as the symbol for: