Uniform Resource Name
A URI--that is, a string--which names a resource. URLs in contrast tell us where to find a resource, i.e. its location. RFC 2141 has the URN of "urn:ietf:RFC:2141", and the URL "http://www.ietf.org/rfc2141.txt ". It's this RFC, incidentally, which defines URNs.
Ironically, it's URL is not valid anymore... a typical problem of URLs.
[Is quoting the White Knight too obvious?]
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