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.
The Greek small letter delta (δ) is used in math as a variable. It appears in calculus in epsilon-delta notation.
The capital Greek letter (Ε).
(ε, δ) "epsilon delta" definitions of limitsGrant Sanderson (3Blue1Brown)