Definition of interconnected water systems determines its level of science and engineering technology. How to summarize existing research and engineering practice scientifically, the research on the definition and classification of interconnected water systems is of great value in theoretical and applications. In response to this problem, this article discusses the meaning and important role of defining the term based on formal logic methods and norms combined with an analysis of the main problems associated with the current term definitions used in the water conservancy industry. Then, we systematically demonstrated the necessity and scientificity of applying an interconnected water system instead of a river and lake water system connection and analysed the various problems that exist in association with the current definition of an interconnected water system. On this basis, the following definition of an interconnected water system is given: "science and technology that study the formation of connected water system channels, the evolution of water systems, and the impact of water body transfers"; interconnected water system engineering is herein defined as "a project that constitutes water transfer channels among water systems through water engineering techniques, rivers and lakes". Then, the connotations and classifications of these two definitions are discussed. These definitions are more scientific, concise and reasonable than the currently used definitions, and more importantly, each definition can scientifically handle the relationship between the adaptability of the concept definition and the completeness of the classification.