Uniform Resource Identifier.
Compact alphanumeric string that identifies resources -physical or abstract- on the web univocally. The difference with a Uniform Resource Locator (URL) is its invariability in the reference of resources. [TSI-RPSI Application Guide]
It is a short string of characters that unambiguously identifies a resource (service, page, document, e-mail address, encyclopedia, etc.). These resources are usually accessible on a network or system. URIs can be uniform resource locators (URL), Uniform Resource Name (URN), or both (URL + URN). [CTIC Foundation]