*Definition [#g699f94f]
-Let  (X, τ) be a topological space. A subset S ⊆ X is called semi-open if there exists a open set O such that O ⊆ S ⊆ Cl(O).

*Property [#h447202c]
-Every semi-open set is [[sg-open>semi-generalized open]] but the converse is not true.

*Reference [#h9df4593]
-Bhattacharyya Paritosh, Lahiri B. K., ''Semigeneralized closed sets in topology.'', Indian J. Math. 29 (1987), no. 3, 375-382 (1988).