Meagre set

In the mathematical fields of general topology and descriptive set theory, a meagre set (also called a meager set or a set of first category) is a set that, considered as a subset of a (usually larger) topological space, is in a precise sense small or negligible. The meagre subsets of a fixed space form a -ideal of subsets; that is, any subset of a meagre set is meagre, and the union of countably many meagre sets is meagre.