Health numeracy has often been overshadowed by health literacy, either ignored completely or identified simply as a subset of health literacy. Only now are researchers beginning to realize the importance of health numeracy as a separate entity. One of the first steps in this evolution is to establish a distinct definition for health numeracy, something that has not been addressed in the literature to date. This paper proposes such a definition, as well as a set of clarifying categories in hopes of helping researchers both to advance the field of health numeracy and to focus their topics within the realm of health numeracy.