The aim of this article is to both theoretically and empirically define the concept of Library 2.0. Design/methodology/approach Written answers to 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 This 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 (if applicable) The model provides a basis for framing Library 2.0 as a research object and to map central themes of future research. Practical implications (if applicable) 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 crowdsourcing approach of defining the concept. Library 2.0 and provides basis for discussing the future evolution of the notion and its implications to library and information science research and library practises.