This chapter aims to provide an introduction to the motivations that lay behind the development of the definition, its utility and its meaning. It utilizes the process of evaluating whether the definition is taxonomically appropriate as a means of unraveling the intended meaning of the definition. The International Association for the Study of Pain (IASP) was founded in 1973 and its first taxonomy of pain terms was published in 1979. The IASP’s Taxonomy was intended to provide a “minimum standard vocabulary for members of different disciplines who work in the field of pain”. Taxonomy often concerns the taxonomic features of things that already fall under an informal classification. The function of a taxonomic definition of pain is to express clearly and succinctly the features that pick out all and only pains. The chapter explains why a taxonomic definition has little utility for clinicians and pain scientists, and focuses on the thinking that underpins the wording of the definition and note.