A Lindelöf space is a topological space in which every open cover has a countable subcover. A closed discrete subset of a Lindelöf space must be countable: cover each of its points by an open set meeting it at only that point and add the open complement. In a metrizable space, the Lindelöf property is equivalent to topological separability.
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A **Lindelöf space** is a concept from topology, a branch of mathematics that deals with the properties of space that are preserved under continuous transformations. Specifically, a topological space is termed **Lindelöf** if every open cover of the space has a countable subcover.