What is a URI?

In the context of recent discussions is anyone on list able to give me a 
succinct and precise definition of what a URI is?

To head off some of the likely reflex responses to such a seemingly simple 
question I would point out that the definitions in RFC 2396 of both 
"resource" and "identifier" make use of the term "identity" whose precise 
meaning is also unclear.

A precise definition of the term URI would be interesting but I wonder if we 
should, pragmatically, recognise that a URI is the Swiss Army Knife of the 
Web. By that I mean that a URI can be used for many purposes and that an 
attempt to too narrowly define legitimate uses of a URI and precisely what a 
URI is and does is potentially counterproductive and unnecessarily 
restrictive of future development of the Web.

Andrew Watt
"XHTML 2.0 - the W3C leading the Web to its full potential ... to implement 
yesterday's technology tomorrow"