\begin{definition}[Definition:Closed Ball/P-adic Numbers/Center]
Let $p$ be a prime number.
Let $\struct {\Q_p, \norm {\,\cdot\,}_p}$ be the $p$-adic numbers.
Let $a \in R$.
Let $\epsilon \in \R_{>0}$ be a strictly positive real number.
Let $\map { {B_\epsilon}^-} a$ be the closed $\epsilon$-ball of $a$.
In $\map { {B_\epsilon}^-} a$, the value $a$ is referred to as the '''center''' of the closed $\epsilon$-ball.
\end{definition}