Definitions of health in terms of some kind of balance form a category of their own within the sphere of health definition. Such definitions have their roots in the beginnings of scientific medicine, and popular versions are common among lay people. It has even been claimed that balance is fundamental to health for all species. Several present-day definitions of health in terms of balance are presented here. Particular attention is given to the call for a definition of health applicable to both humans and animals within the One Health approach, involving human medicine, veterinary medicine and ecology. Definitions in terms of balance have been suggested but none has been thoroughly analysed with regard to its suitability. There are therefore three concerns in this paper. The first is to introduce versions of the category of balance, as a first step towards a nomenclature of health definitions. The second is to analyse the claim made recently that balance is a universal criterion of health in all species including humans. The third is to ascertain whether any of the versions discussed is suited to the One Health approach.