What is a namespace (Namensraum)?

A namespace is the scope, such as a company, a state or a professional community, within which the meaning of a designation such as an identifier is defined; inside it, subjects and resources are named unambiguously by at least one identifier.

Two systems that exchange identities must either share a namespace or map between them, which is what identity linking and mapping do.

Go deeper:

• Namespace (Wikipedia) — why a name is only unique within a scope.