An empire involves the extension of a state's sovereignty over external territories and variety of different ethnic groups. The term "empire" in this context (not necessarily a state ruled by an emperor) does not have a precise definition, but is generally applied to political entities that are considered to be especially large by the standards of their time and that have acquired a significant part of their territory by conquest. This article provides a list of the largest empires in world history, but the list is not and cannot be definitive since the decision about which entities to consider as "empires" is difficult and fraught with controversy.