Namespace List
Here is a list of all documented namespaces with brief descriptions:

__cxxabiv1 |
__gnu_cxx | GNU extensions for public use
__detail | Implementation details not part of the namespace __gnu_cxx interface
typelist | GNU typelist extensions for public compile-time use
__gnu_debug | GNU debug classes for public use
__gnu_internal | GNU implemenation details, not for public use or export. Used only when anonymous namespaces cannot be substituted
__gnu_parallel | GNU parallel code for public use
__gnu_pbds | GNU extensions for policy-based data structures for public use
__gnu_sequential | GNU sequential classes for public use
abi | The cross-vendor C++ Application Binary Interface. A namespace alias to __cxxabiv1, but user programs should use the alias 'abi'
simd_abi |
std | ISO C++ entities toplevel namespace is std
__debug | GNU debug code, replaces standard behavior with debug behavior
__detail | Implementation details not part of the namespace std interface
__exception_ptr |
__parallel | GNU parallel code, replaces standard behavior with parallel behavior
_V2 |
chrono | ISO C++ 2011 namespace for date and time utilities
decimal | ISO/IEC TR 24733 Decimal floating-point arithmetic
experimental | Namespace for features defined in ISO Technical Specifications
filesystem | ISO C++ 2017 namespace for File System library
literals | ISO C++ inline namespace for literal suffixes
chrono_literals |
placeholders | ISO C++ 2011 namespace for std::bind placeholders
pmr |
regex_constants | ISO C++ 2011 namespace for options and flags used with std::regex
rel_ops | The generated relational operators are sequestered here
this_thread | ISO C++ 2011 namespace for interacting with the current thread
tr1 | ISO C++ TR1 entities toplevel namespace is std::tr1
__detail | Implementation details not part of the namespace std::tr1 interface
tr2 | Namespace for non-standard "TR2" extensions
__detail | Implementation details not part of the namespace std::tr2 interface