##  [Open Set](/open-set) 

  ##  [Open Set](https://puremath.quantumdictionary.io/open-set-0) 

  

 [![Pure Mathematics Dictionary](/sites/default/files/styles/large/public/2026-01/Pure%20Mathematics.png.webp?itok=5pZnFQ59)](/topic-specific-dictionaries/mathematics-logic/pure-mathematics)

- Mathematics & Logic -

**Pure Mathematics Dictionary**

 







 

 

 

 



 

 

 

 

Definition

A subset U of a topological space (X, τ) that belongs to the topology τ; equivalently, for every point x in U there exists a neighborhood (an open set from τ) contained in U.