Any XML element or attribute may belong to a namespace, that is, a set of elements and attributes that belong to a single document type.
Most XML document types are defined by a schema that not only enumerates all the element and attribute names in that document type, but also defines the rules for structuring the document: which elements may be inside which other elements, and which attributes may occur in which elements.
Throughout the present document, we use the term namespace URI to mean a URI that uniquely identifies a namespace.
For example, the XHTML 1.0 document type is associated with this namespace URI:
In this case, there actually is a DTD (Document Type Definition) schema located on the Web at that URI. However, a namespace URI may be only an identifier, and it may or may not be function as a location.