NamespaceURI

NamespaceURI is the Uniform Resource Identifier that identifies the namespace associated with an XML element or attribute. It is used to distinguish names that share the same local part but belong to different vocabularies by qualifying them with a namespace.

In XML, the expanded name of a node consists of a namespace URI and a local name. The namespace URI is declared and mapped to prefixes via xmlns declarations, such as xmlns:ns="http://example.org/ns" or xmlns="http://example.org/ns" for the default namespace. The default namespace applies to element names but not to unprefixed attribute names; attributes that have no explicit prefix are not placed in the default namespace. The combination of NamespaceURI and local name forms a globally distinct identifier for a node.

In programming interfaces such as the Document Object Model (DOM), the NamespaceURI property exposes this value. Node.namespaceURI returns the namespace URI as a string, or null if the node does not participate in a namespace. For HTML documents, behavior can vary by parser and DOM implementation; elements may be associated with the XHTML namespace or may expose null when namespaces are not used.

Practical use includes XML processing and XPath evaluation, where correct namespace mappings are required for queries to match. When serializing or querying XML, NamespaceURI helps ensure that names from different vocabularies are handled unambiguously.