What does "epsilon" mean?

The fifth letter of the Greek alphabet, ε or Ε, preceded by delta (Δ, δ) and followed by zeta (Ζ, ζ).

Epsilon

Common synonyms include: arbitrary small quantity, empty transition, epsilon move, epsilon symbol, epsilon value, epsilon-delta.

/ɛpˈsaɪ.lən/ noun, slang

noun, slang ·Moderate ·High school level

Noun

1
The fifth letter of the Greek alphabet, ε or Ε, preceded by delta (Δ, δ) and followed by zeta (Ζ, ζ). countable, uncountable
2
the 5th letter of the Greek alphabet wordnet
3
In IPA, the phonetic symbol ɛ that represents the open-mid front unrounded vowel. countable, uncountable
4
An arbitrarily small quantity. countable, uncountable
5
An arbitrarily small quantity.; A small child. colloquial, countable, uncountable

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6
Something negligible or insignificant. colloquial, countable, uncountable
"Yes, we have to convert all the symbol names to upper case at startup, but that’s epsilon."
7
The percentage change in an option value with respect to the underlying dividend yield. countable, uncountable

"But forecasters say it is likely Bermuda will be sideswiped by Epsilon when it makes its closest pass sometime late Thursday."

From Ancient Greek ἒ ψιλόν (è psilón, “simple Ε”).