---
tag: 02BH3MCBQB
kind: concept
name: Open set
msc: [54A05]
sources:
  - url: https://en.wikipedia.org/wiki/Open_set
    locator: lead section
same_as:
  wikidata: Q213363
  wikipedia: Open set
---

## Summary

An open set is a member of a topology.
In a metric space, a set is open when each of its points has a small ball around it that stays in the set.