A namespace is a container that provides context for identifiers, within which names are unique. In many implementations, identifiers can be disambiguated between namespaces by prepending the identifier with the namespace, separated by a delimiter such as a period (.) in java and c#, double-colon (::) in c++ or backslash (\) in php-5.3.

For many programming languages, namespace is a context for their identifiers. In an operating system, an example of namespace is a directory. Each name in a directory uniquely identifies one file or subdirectory, but one file may have the same name multiple times.

As a rule, names in a namespace cannot have more than one meaning; that is, different meanings cannot share the same name in the same namespace. A namespace is also called a context, because the same name in different namespaces can have different meanings, each one appropriate for its namespace.

Following are other characteristics of namespaces:

• Names in the namespace can represent objects as well as concepts, be the namespace a natural or ethnic language, a constructed language, the technical terminology of a profession, a dialect, a sociolect, or an artificial language (e.g., a programming language).
• In the Java programming language, identifiers that appear in namespaces have a short (local) name and a unique long "qualified" name for use outside the namespace.
• Some compilers (for languages such as C++) combine namespaces and names for internal use in the compiler in a process called name mangling.

PHP

Namespaces were introduced into PHP from version 5.3 onwards. In PHP, a namespace is defined with a namespace block.

namespace phpstar {
    class fooBar {
        public function foo() {
            echo 'hello world, from function foo';
        }

        public function bar() {
            echo 'hello world, from function bar';
        }
    }
}

XML

In XML, the XML namespace specification enables the names of elements and attributes in an XML document to be unique. Using XML namespaces, XML documents may contain element or attribute names from more than one XML vocabulary.

Python

In Python, namespaces are defined by the individual modules, and since modules can be contained in hierarchical packages, then name spaces are hierarchical as well. In general when a module is imported then the names defined in the module are defined via that module's name space, and are accessed in from the calling modules by using the fully qualified name.

.NET

All .NET Framework classes are organized in namespaces. When referencing a class, one should specify either its fully qualified name, which means namespace followed by the class name,

C++

In C++, a namespace is defined with a namespace block.

namespace abc {
    int bar;
}