In the Jabber protocol, an element <x/> that may be added to a <message/> or <presence/> element to convey additional XML-structured information. Jabber clients that do not recognize any given extension tag simply ignore it.

The <x/> element always specifies the namespace of the extension. Standard Jabber extensions all have namespaces beginning with the prefix "jabber:x:". Standard extensions can be used to send out-of-band data references or roster items with a message, or to specify message timing or envelope information.