What's a namespace?

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Randall Fowle writes: A namespace is a collection of element and attribute names identified by a Uniform Resource Identifier reference. The reference appears in the root element as a value of the xmlns attribute. For example, the namespace reference for an XML document with a root element x might appear like this: .

More than one namespace may appear in a single XML document, to allow a name to be used more than once. Each reference can declare a prefix to be used by each name, so the previous example might appear as , which would nominate the namespace for the `spc' prefix: Mr. Big.

The reference does not need to be a physical file; it is simply a way to distinguish between namespaces. The reference should tell a person looking at the XML document where to find definitions of the element and attribute names using that particular namespace.