early 13c., from Old French empereor "emperor, leader, ruler" (11c.; accusative; nominative emperere; Modern French empereur), from Latin imperatorem (nominative imperator) "commander, emperor," from past participle stem of imperare "to command" (see empire).
Originally a title conferred by vote of the Roman army on a successful general, later by the Senate on Julius and Augustus Caesar and adopted by their successors except Tiberius and Claudius. In the Middle Ages, applied to rulers of China, Japan, etc.; non-historical European application in English had been only to the Holy Roman Emperors (who in German documents are called kaiser), from late 13c., until in 1804 Napoleon took the title "Emperor of the French."
In England, the name replaced native Weodmonað "weed month." Traditionally the first month of autumn in Great Britain, the last of summer in the U.S.