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. For example, first the Portuguese Empire, then the Spanish Empire and later the British Empire were called "the empires on which the sun never sets", because of their vast territories and possessions around the globe. 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", such as the concept of American Imperialism, is difficult and fraught with controversy.