How entities, the things that exist, are defined and categorised affects all aspects of environmental management including technical descriptions, quantitative analyses, participatory processes, planning, and decisions. Consequently, ambiguous definitions and wrongly assigning entities to categories, referred to as category mistakes, are barriers to effective management. Confusion caused by treating the term ‘biodiversity’ variously as the property of an area, the biota of an area, and a preferred end state (a value) – quite different categories of entities – is one example. To overcome such difficulties, we develop and define four entity categories – elements, processes, properties, and values – and two derived categories – states and systems. We argue that adoption of these categories and definitions will significantly improve environmental communication and analysis, and thus strengthen planning and decision-making.