*Definition [#yd75e5aa]
-A subset S of a topological space (X,τ) is called η-open if S is the union of a [[δ-closed sets>δ-closed]].

*Property [#a55893d3]
-Every η-open set is [[∨-set]].

*Reference [#ef08ce5c]
-Dontchev, J.; Popvassilev, S.; Stavrova, D., ''On the η-expansion topology for the co-semi-regularization and mildly Hausdorff spaces.'', (English summary), Acta Math. Hungar. 80 (1998), no. 1-2, 9-19.