Encyclopædia Britannica, Ninth Edition/Emperor
According to mediæval theory, there was and could be only one emperor in the world, the direct vicegerent of God, who represented the unity of mankind and of the Christian people on its temporal side as the pope did on its Spiritual. Hence during those ages the Western monarch and Western writers did not admit in principle, though they sometimes recognized in fact, the title of the emperor who reigned at Constantinople; and the Easterns in like manner denied the existence of an emperor in the West, and maintained that the heads of the Holy Roman Empire were merely German intruders. In spite, however, of the universal acceptance of the theory above mentioned, the title of emperor was one which other princes seem to have hankered after. In 1053 Ferdinand the Great of Castile, in the pride of his victories over the Moors, assumed the style of Hispanicæ imperator, but was forced by the remonstrances of the emperor Henry III. to abandon it. In the 12th century it was again assumed by Alphonso VII. of Castile, but not by any of his successors. In England the Anglo-Saxon kings frequently used the term basileus, and sometimes also imperator, partly from a desire to imitate the pomp of the Byzantine court, partly in order to claim a sovereignty over the minor kingdoms and races of the British isles corresponding to that which the emperor was held to have over Europe generally (see Freeman, Norman Conquest, vol. i., Appendix, who however attaches too much importance to this English use).
In comparatively modern times, the title of emperor has been taken by the monarchs of Russia (Vassili, about 1520, his predecessors at Moscow having been called Great Dukes of Muscovy, and the title of Czar or Tsar being apparently a Slavonic word for prince, not related to Cæsar), France (Napoleon Bonaparte in 1804, Louis Napoleon Bonaparte in 1853), Austria (1805), Brazil (1822), Germany (December 31, 1870), Great Britain and Ireland in respect of the Indian dominions of the crown (1876). Usurpers who have reigned in Hayti, a certain Augustin Iturbide who 1822) became ruler of Mexico after the revolt against Spain, and the archduke Maximilian of Austria during his short tenure of power in Mexico, also called themselves emperors; and modern usage applies the term to various semi-civilized potentates, such as the sovereigns of China and Morocco. It can, therefore, hardly be said that the name has at present any deﬁnite descriptive force, such as it had in the Middle Ages, although its associations are chiefly with arbitrary military power, and it is vaguely supposed to imply a sort of precedence over kings. In the cases of Germany, Austria, and Britain in respect of India, it may perhaps be taken to denote that general over-lordship which their sovereigns exercise over minor princes and over their various territories, and which is distinct from their position as sovereigns of one or more particular kingdom or kingdoms, the German emperor being also king of Prussia, as the emperor of Austria is king of Hungary, and the empress of India queen of Great Britain and Ireland.
See Selden, Titles of Honour, Bryce, Holy Roman Empire; Sir E. Colebrooke, “On lmperial and other Titles,” in the Journal of the Royal Asiatic Society, 1877.