Definition of HTML "namespace"
A namespace is an attribute associated with the name of some object, such as a node in an HTML document, that distinguishes the type of object represented by the name from other types of objects with the same name in other namespaces. For example, in
<head><title>Heading Title</title></head>
title is the name of an element (the title element) in HTML while in
<a href="..." title="link title" ...>
title is the name of a title attribute on an HTML <a> tag.
On this site, the notation <a href> refers to the href attribute in the html:a element namespace, where html: is the namespace prefix for the HTML namespace.
See the HTML Namespace Tutorial for more information on namespaces in HTML 5.