Syntax:

<namespace>a_namespace</namespace>

 [Required for a language definition]

It specifies the namespace of this language. It is suggested to be an URL. The last part will be considered as a shortcut. Thus, the last part is better to be identifiable

Example, here is the ZUL namespace and it also defines a shortcut.

<namespace>http://www.zkoss.org/2005/zul</namespace>