*Definition [#k6d2c019]
-A subset S of a topological space (X,τ) is called a semi-Difference set (in short sD-set) if there are two [[semi-open]] sets U, V in X such that &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20U%5cneq%20X%20%5cmbox%7b%20and%20%7dS=U%5csetminus%20V%20%5c%5d%7d%25.png);.

*Reference [#d5bb794d]
-Athisaya Ponmani, S.; Lellis Thivagar, M., ''Another form of separation axioms.'' ,  (English) [J] Methods Funct. Anal. Topol. 13, No. 4, 380-385 (2007).