epsilon; Ε, ε +

(Greek: the fifth letter of the Greek alphabet; Ε, ε)

1. The fifth letter and second vowel of the Greek alphabet (Ε, ε) equivalent to the English short e.
2. As a numeral, it refers to to 5; relating to the fifth member of a particular ordered set.
3. In mathematics, an arbitrarily small quantity, used to indicate that a given quantity is small, or close to zero.
In astronomy, the name for Uranus' most distant and most visible ring.
epsilon toxin
A bacterial toxin produced by clostridium perfringens or anaerobic Gram-positive rod bacterium that produces epsilon toxin.

It causes intense abdominal cramps and diarrhea that begins 8-22 hours after consumption of foods containing large numbers of these bacteria.

epsilon-aminocaproic acid
A synthetic substance used to correct an overdose of certain fibrinolytic agents (causing the dissolution of fibrin or blood-clotting by enzymatic action or a chemical that is produced by living cells which is a biochemical catalyst).

It is also useful in treating excessive bleeding due to increased fibrinolytic activity in the blood.

Referring to a method or proof in calculus involving arbitrarily small numbers.
epsilon-neighborhoods, epsilon neighborhood
1. The set of all points whose distance from a given point is less than some specified number epsilon.
2. The term epsilon neighborhood (or ε-neighborhood) is often used to emphasize that ε is arbitrarily small.

You can see all of the letters of the Greek Alphabet on this page.