Definition:Sphere/Metric Space/Radius
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Definition
Let $M = \struct{A, d}$ be a metric space or pseudometric space.
Let $a \in A$.
Let $S_\epsilon \paren{a}$ be the $\epsilon$-sphere of $a$.
In $S_\epsilon \paren{a}$, the value $\epsilon$ is referred to as the radius of the $\epsilon$-sphere.
Linguistic Note
The plural of radius is radii, pronounced ray-dee-eye.
This irregular plural form stems from the Latin origin of the word radius, meaning ray.
The ugly incorrect form radiuses can apparently be found, but rarely in a mathematical context.
Caution
It should be noted that the radius is not intrinsic to the $\epsilon$-sphere, so that the radius of a sphere is ambiguous.