Wikipedia definition: "Library 2.0 is a loosely defined model for a modernized form of library service that reflects a transition within the library world in the way that services are delivered to users. The focus is on USER-CENTERED change and participation in the creation of content and community."
"What makes a service Library 2.0? Any service, physical or virtual, that successfully reaches users, is evaluated frequently, and makes use of customer input is a Library 2.0 service.-- Michael Stephens (http://lj.libraryjournal.com/ 2010/05/technology/library-2-0/)