URN

A Uniform Resource Name is a Uniform Resource Identifier or URI that use the "urn" scheme, and does not connote availability of the identified resource.

Uniform Resource Names (URNs) are intended to serve as persistent, location-independent, resource identifiers and are designed to make it easy to map other namespaces (which share the properties of URNs) into URN-space. Therefore, the URN syntax provides a means to encode character data in a form that can be sent in existing protocols, transcribed on most keyboards, etc.