Disk

A 2D version of Open Ball.

A disk $D_{ϵ}(p)$ of radius $ϵ>0$ is the set of all 2D points of distance at most $ϵ$ from $p$ $D_{ϵ}(p)=\{x\in {\mathbb{R}}^2:||x-p||<ϵ\}$