XML Schema Complete Reference, The

3.2. Namespace Components

Namespaces enable you to create unique names that appear in an XML document. You associate a URI with the names that you use in your document. When a name is associated with a URI, it is called a “qualified name” and is said to be in that namespace. You can also have names in your document that are not in any namespace. These names are called “unqualified.” You associate names in your document with a namespace by using a prefix. A prefix is an arbitrary string of characters that acts as shorthand for the namespace URI so that names are not overly long. The prefix is also optional, because you can declare a namespace to be a default, meaning all unprefixed element names are in that namespace by default.