The term 'environmental security' is widely used in the academic, policy, and activist communities, but there is little agreement about what it means or how it should be operationalized. This chapter argues that a concise, narrowly focused, and systematic definition would be beneficial for the purposes of policy, research, and environmental rescue. It begins with a brief discussion of how rhetorical and analytical ambitions have been interwoven in the environmental movement, resulting in a 'gray language of environmental politics'. While this is useful in certain ways, it also poses a number of problems that are exemplified in the term 'environmental security'. The chapter then provides a critical overview of the various ways in which the term is used. A concise definition is proposed, explained, and defended on the grounds that it provides the basis for both a useful research agenda and a fruitful dialogue with security specialists. Many of the ideas currently associated with the term 'environmental security' that are not contained in the narrow definition are important. The chapter concludes with an argument for situating these ideas in the broader context of research and debate on world order.