According to many authors (e.g., Kelley 1955, p. 112; Joshi 1983, p. 162; Willard 1970, p. 99) a normal space is a topological
space in which for any two disjoint closed sets there are two disjoint open sets and
such that
and .
Other authors (e.g., Cullen 1968, p. 118) define the notion differently, using separation axioms.