Let XXX be a topological space. The subset A⊂XA \subset XA⊂X is closed if its complement X∖AX\setminus AX∖A is open. Equivalently, if XXX is a metric space, then a subset A⊂XA \subset XA⊂X is closed if and only if it contains all of its limit points.
Wikidata ID: Q320357