Definition:Closed Ball/Metric Space/Radius

Definition
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.