9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

9.8.1 General [basic.namespace.general]

A namespace is an optionally-named entity whose scope can contain declarations of any kind of entity.
The name of a namespace can be used to access entities that belong to that namespace; that is, the members of the namespace.
Unlike other entities, the definition of a namespace can be split over several parts of one or more translation units and modules.
[Note 1: 
A namespace-definition is exported if it contains any export-declarations ([module.interface]).
A namespace is never attached to a named module and never has a name with module linkage.
— end note]
[Example 1: export module M; namespace N1 {} // N1 is not exported export namespace N2 {} // N2 is exported namespace N3 { export int n; } // N3 is exported — end example]
There is a global namespace with no declaration; see [basic.scope.namespace].
The global namespace belongs to the global scope; it is not an unnamed namespace ([namespace.unnamed]).
[Note 2: 
Lacking a declaration, it cannot be found by name lookup.
— end note]