A variant form of the Greek letter epsilon commonly used to represent small quantities or error terms in analysis and physics.
Examples
Small positive number in limit definition
\forall \varepsilon > 0, \exists N \in \mathbb{N} : |x_n - L| < \varepsilon
Error term in approximation
f(x) = ax + b + \varepsilon
Small neighborhood in topology
B_\varepsilon(x) = \{y \in X : d(x,y) < \varepsilon\}