---
tag: 0YE5868HZB
kind: concept
name: Open ball
msc: ["54E35"]
sources:
  - url: https://en.wikipedia.org/wiki/Ball_(mathematics)
    locator: In general metric spaces
---

## Summary

An open ball in a metric space is the set of all points at distance strictly less than a given radius from a given centre.
Open balls are the basic open sets of a metric space.