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 C, D there are two disjoint open sets U and V such that C⊆U and D⊆V. Other authors define the notion differently, using separation axioms.

Hilbert cube | Tietze's extension theorem | Tychonoff plank | Urysohn's lemma