Purpose - The aim of this paper is to define both theoretically and empirically the concept of Library 2.0. Design/methodology/approach - Written answers to the question "What is Library 2.0?" given by practitioners and researchers (n = 29) interested in Library 2.0 issues were analyzed by using co-word analysis to map the underlying elements of the concept. Findings - The study resulted in a model of Library 2.0, containing seven building-blocks of the phenomenon: interactivity, users, participation, libraries and library services, web and web 2.0, social aspects, and technology and tools. Research limitations/implications - The model provides a basis for framing Library 2.0 as a research object and to map central themes of future research. Practical implications - A comprehensive model enables both researchers and practitioners to frame the phenomenon more clearly, evaluate existing and planned services and their proximity to what is Library 2.0. Originality/value - Unlike earlier proposals for a definition of the notion Library 2.0, the present study presents an empirical and consensual crowd-sourcing approach of defining the concept Library 2.0 and provides basis for discussing the future evolution of the notion and its implications for library and information science research and library practices. [ABSTRACT FROM AUTHOR]