 A topological space "X" is a regular space if, given any closed set "F" and any point "x" that does not belong to "F", there exists a neighbourhood "U" of "x" and a neighbourhood "V" of "F" that are disjoint.Concisely put, it must be possible to separate "x" and "F" with disjoint neighborhoods.A T3 space or regular Hausdorff space is a topological space that is both regular and a Hausdorff space.