Symbols:Greek/Epsilon/Arbitrarily Small Positive Quantity

Arbitrarily Small Strictly Positive Real Number
Many a proof in analysis will famously start:
 * "Let $\epsilon > 0$ ..."

where it is frequently left unstated that $\epsilon$ is a real number, arbitrarily small.

Also denoted as
While $\epsilon$ is common, so is $\varepsilon$.

The symbols are, in general, interchangeable.

Some writers prefer $\epsilon$ and some prefer $\varepsilon$.

On the preferred symbol is $\epsilon$.