Fatigue is probably the most common symptom of illness affecting sufferers of both acute and chronic conditions. However, confusion surrounds the definition and use of the term fatigue. As with many other nursing concepts, it is a word that is commonly used in colloquial language. This concept analysis aims to identify the attributes that are essential to the concept of fatigue, and to distinguish between its colloquial and its nursing usage by following the strategy suggested by Walker and Avant (1995, Strategies for Theory Construction in Nursing, Appleton Lange, London). A review of the literature identifies nursing uses of the term fatigue which reflect and conflict with colloquial uses. Defining attributes, demonstration cases, antecedents, consequences and empirical referents are identified before a definition of fatigue is developed and proposed for nursing. This clarification of the phenomenon has particular relevance for clinical nursing, future research and the development of fatigue theories.