"Epsilon" (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, , in LaTeX), which has its origins in Medieval Greek.
In mathematical notation, the minuscule open e symbol ɛ (\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 or falsely recognized as the lunate version Σ.
Symbol
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 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.
"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.