Let $X$ be a topological space. An open cover of $A$ is a collection of open subsets of $X$ such that $A$ lies in the union of the elements of the collection. That is, an open covering is a collection ${G_\lambda}{\lambda\in\Lambda}$ of subsets of $X$ such that $A\subset \bigcup\limits{\lambda\in\Lambda} G_\lambda$.
Wikidata ID: Q41509136