An ordered pair \((X, \tau)\), where \(X\) is a set and \(\tau\) is a collection of subsets of \(X\) satisfying:
- The empty set (\(\emptyset\)) and \(X\) belong to \(\tau\)
- Any arbitrary (in)finite union of members of \(\tau\) still belongs to \(\tau\)
- The intersection of any finite number of members of \(\tau\) still belongs to \(\tau\)