A lowercase Greek letter commonly used to represent small quantities, error terms, or membership in set theory.
Examples
Small positive number in limit definition
\forall \epsilon > 0, \exists N \in \mathbb{N} : |x_n - L| < \epsilon
Error term in approximation
f(x) = ax + b + \epsilon
Set membership with small parameter
B_\epsilon(x_0) = \{x \in \mathbb{R} : |x - x_0| < \epsilon\}