\begin{definition}[Definition:Open Ball/Center]
Let $M = \struct {A, d}$ be a metric space or pseudometric space.
Let $a \in A$.
Let $\map {B_\epsilon} a$ be the open $\epsilon$-ball of $a$.
In $\map {B_\epsilon} a$, the value $a$ is referred to as the '''center''' of the open $\epsilon$-ball.
\end{definition}