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.

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:

• A very small quantity in mathematics.
• The limit ordinal of the sequence in set theory.
• The precision of a numeric data type in computing.
• The empty string in computer science.
• The Levi-Civita symbol in mathematics.
• Set membership in mathematics (often written ∈ instead of ε), especially in older texts.
• The permittivity of a medium in physics.
• In automata theory, a transition that involves no shifting of an input symbol.

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.