Epsilon (upper case Ε, lower case ε) is the fifth letter of the Greek alphabet. In the system of Greek numerals it has a value of 5. "E psilon" ("simple e") was coined to distinguish the letter from αι, which by the medieval period was pronounced the same way.

Epsilon is the lowest and least intelligent caste in Aldous Huxley's dystopic satire Brave New World.

The lower-case letter ε is used as the symbol for:

• In mathematics, a small positive quantity, usually denoted ε; see limit.
  
• In set theory, the limit ordinal of the sequence .
  
• In computing, the precision of a numeric data type.
  
• In computer science, the empty string.
  
• In mathematics, the Levi-Civita symbol.
  
• In mathematics, set membership (often written ∈ instead of ε), especially in older texts.
  
• The late mathematician Paul Erdős also used the term "epsilons" to refer to children (Hoffman 1998, p. 4).
  
• In physics, the permittivity of a medium.
  
• In automata theory, a transition that involves no shifting of an input symbol.
  
• In astronomy, the fifth brightest (usually) star in a constellation. See Bayer designation.
  

There is a company that has this name:

Epsilon S.A - studio company

Letters that arose from Epsilon include the Roman E.

Epsilon in Malaysian Pale is the name of the second solo album released by Tangerine Dream leader Edgar Froese in 1975.

Epsilon is also an Australian light-powered female-looking robot that fought Pluto in an episode of Astro Boy.

Epsilon Eridani III is the planet that the spacestation Babylon 5 orbited in the eponymous sci-fi television series.