This reflection paper on existing e-health definitions is based on the work to define e-health for the purpose of national monitoring activities in Norway. Our aim is to encourage reflection on the challenges in defining e-health, and on the importance of definitions to succeed in monitoring and indicator work. The study was conducted by the authors from the Norwegian Centre for e-health research as part of an assignment from the Norwegian Directorate of e-health to give an account of possible concepts for a Norwegian e-health monitor. We did a web-based literature search and a hands-on search in policy papers from WHO, OECD and the EC. The results show there is growth in the interest for e-health, and that existing definitions can be grouped in three main categories: definitions based in a) target groups, b) application areas and c) generic aims. We conclude e-health definitions need to be fluid to encompass the complexity of the socio-technical context and the rapid changes in the field. Hence the choice of definition type always has to be made with regard to context, evaluation design and objectives of monitoring.