Wiktionary
open ball
n. (context topology analysis restricted to metric spaces English) The set of all points in a metric space whose distance from a given point ("the open ball's center") is less than a given length ("the open ball's radius").
Usage examples of "open ball".
Out in the park, beyond the wall of the hedge, moonlight bathed the open ball fields and play areas with silver brightness.
She let him be, striding across the open ball diamonds toward the service road and the hedgerow that marked the boundary between the park and her backyard.