This chapter proposes a definition for legal fiction. The problem of definition has divided scholars into multiple camps and held back progress. The definition proposed here is a compromise which seeks to preserve precision while covering the true range of fictitious devices. My definition is composed of two limbs: Hard Fiction and Soft Fiction. In developing this definition, I address a range of theoretical issues. I rebut arguments by Vaihinger and Kelsen that fictions do not exist. I discuss fictions from a linguistic perspective. I discuss the relationship between fictions and rules, and between fictions and counter-factuals. Also explored are contributions to the definitional debate by Fuller, Olivier, Ross, Chiassoni, Del Mar and earlier thinkers. The Chapter concludes with an application of the proposed definition in two case studies: the reasonable man and volenti non fit injuria. I argue the latter is a Soft Fiction and the former is not a fiction.