An empire involves the extension of a state's sovereignty over external territories and a 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.
List of largest empires Wikipedia
Both what constitutes an empire and the calculation of the land area of a particular empire are controversial subjects. Rein Taagepera has defined an empire as "any relatively large sovereign political entity whose components are not sovereign" and its size as the area over which the empire has some undisputed military and taxation prerogatives.
For context, note that the total land area of the Earth is 148,940,000 km2 (57,500,000 sq mi). This is a list of empires whose greatest extent exceeded 2% of that area.