*Definition [#xdad81b1]
-A topological space (X,τ) is called semi-D_1 if for &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20x%2cy%5cin%20X%2c%5c%20x%5cneq%20y%20%5c%5d%7d%25.png); , there exists a [[sD-sets>semi-Difference set]] of X one containing x but not y and the other containing y but not x.

*Property [#q889276b]
-If (X,τ) is semi-D_1, then it is [[semi-D_0]].

*Reference [#w02c1a24]
-Athisaya Ponmani, S.; Lellis Thivagar, M., ''Another form of separation axioms.'' ,  (English) [J] Methods Funct. Anal. Topol. 13, No. 4, 380-385 (2007).