The term used to describe a Uniform Resource Identifier (URI) that does not change. This enables resources that have been linked to, bookmarked, or referenced in other ways to remain valid links for many years to come. A cool URI is considered to be a foundation requirement for the Semantic Web. As described by Tim Berners-Lee in "Cool URIs don't change" (1998), URIs should be designed with three things in mind: simplicity, stability and manageability.
See also "What Is The Semantic Web?" in the Did You Know...? section of Webopedia.