拓扑空间(X,τ)，A⊆X，称A是无处稠密的（亦称稀疏的，或称A为无处稠密集、稀疏集），当且仅当A的闭包的内部是空集。

In mathematics, a nowhere dense set in a topological space is a set whose closure has empty interior. In a very loose sense, it is a set whose elements are not tightly clustered (as defined by the topology on the space) anywhere. The order of operations is important. For example, the set of rational numbers, as a subset of R, has the property that the interior has an empty closure, but it is not nowhere dense; in fact it is dense in R. Equivalently, a nowhere dense set is a set that is not dense in any nonempty open set.