---
tag: 0CW52NV5TC
kind: concept
name: Annulus
aliases: [annular-region, ring-domain]
msc: [30A99]
sources:
  - url: https://en.wikipedia.org/wiki/Annulus_(mathematics)
    locator: lead section
same_as:
  wikidata: Q45926
  wikipedia: Annulus (mathematics)
---

## Summary

An annulus is the region between two concentric circles, see the [[definition|0AJ0YVK743]].
In complex analysis, it is the open set $r < |z - c| < R$, and it is the natural domain of a Laurent series.