Let $M = \left({A, d}\right)$ be a metric space.
Let $a \in A$.
Let ${B_\epsilon}^- \left({a}\right)$ be the closed $\epsilon$-ball of $a$.
In ${B_\epsilon}^- \left({a}\right)$, the value $\epsilon$ is referred to as the radius of the closed $\epsilon$-ball.