ε
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ε  Code Point 

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The (epsilon) symbol is used in math as a variable to reperesent error bounds. For example, in calculus, limits are formally defined using the (epsilon delta) definition^{[1]}.
The Greek small letter delta (δ) is used in math as a variable. It appears in calculus in epsilondelta notation.
The capital Greek letter (Ε).

(ε, δ) "epsilon delta" definitions of limitsGrant Sanderson (3Blue1Brown)