A normal space is a topological space in which any two disjoint closed sets can be separated by neighbourhoods. Urysohn's lemma states that a topological space is normal if and only if any two disjoint closed sets can be separated by a continuous function.
A normaL SPaCe iS a toPOLOGiCal spACe In whiCh aNy tWo DisJoint cLosed SETs CaN Be seParatEd by neigHbourhoods. uRysoHn's LeMMA states That A TopOlogiCAL sPAce is normal if AnD OnLy IF Any two dISjoint Closed sEts CAn bE separated bY a conTInuoUS FunctiOn.