This paper discusses available definitions of population and criticise them against some basic rules of ontology. None of the found definition satisfies the requirement of an ontology that supports building consistent public heath databases. Most definitions define population on territorial bases or as reproductive communities. The author argues that populations are systems (its members must be in connection to each other) and individuals forming the population must share a common resource. Proper usage of the definitions may help to build consistent ontology for public health indicators and consistent databases.