A subset A {\displaystyle A} of a topological space X {\displaystyle X} has the property of Baire, or is called an almost open set, if it differs from an open set by a meager set; that is, if there is an open set U ⊆ X {\displaystyle U\subseteq X} such that A △ U {\displaystyle A\bigtriangleup U} is meager. Wikipedia