A topological space fulfilling the T3-separation axiom: 
fulfils the T1-separation axiom and is regular.
According to the terminology of Alexandroff and Hopf (1972), 
-spaces are called Vietoris spaces.
T_3-Space
See also
Hausdorff Axioms, Separation Axioms, T0-Space, T1-Space, T2-Space, T3-Separation Axiom, T4-SpaceThis entry contributed by Margherita Barile
Explore with Wolfram|Alpha
References
Alexandroff, P. and Hopf, H. Topologie, Vol. 1. New York: Chelsea, 1972.Cite this as:
Barile, Margherita. "T_3-Space." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/T3-Space.html