A female emperor is properly termed an empress or an empress regnant. Emperors and empresses reign over empires and are generally conceded to outrank kings. Reigning empresses rarely conquer their own territories but instead inherit from a husband or father.
An empire is a far-flung territory that generally contains several different nations, most often ethnically and linguistically distinct from one another. These nations either answer directly to the ruler or pay tribute while largely ruling themselves. Examples of empires include the 19th century British Empire, the classical-age Roman Empire and the Empire of Japan.
Some historic empresses are Empress Xiao of China, who was forced to abdicate the Qing throne in 1912, and Ethiopian Empress Negista Nagast, who preceded Emperor Haile Selassie III, formerly Prince Rasta Fari.