emperor n.

mid-14c., "territory subject to an emperor's rule;" in general "realm, dominion;" late 14c. as "authority of an emperor, supreme power in governing; imperial power," in Middle English generally of the Roman Empire.

From Old French empire "rule, authority, kingdom, imperial rule" (11c.), from Latin imperium "a rule, a command; authority, control, power; supreme power, sole dominion; military authority; a dominion, realm," from imperare "to command," from assimilated form of in- "in" (from PIE root *en "in") + parare "to order, prepare" (from PIE root *pere- (1) "to produce, procure").

Not etymologically restricted to "territory ruled by an emperor," but used that way. The Empire, meaning "the British Empire," first recorded 1772 (it officially devolved into "The Commonwealth" in 1931); before that it meant the Holy Roman Empire (1670s).

[P]roperly an empire is an aggregate of conquered, colonized, or confederated states, each with its own government subordinate or tributary to that of the empire as a whole. [Century Dictionary] 

Empire as the name of a style (especially in reference to a style of dresses with high waistlines) is by 1860, in reference to the affected classicism prevailing in France during the reign of Napoleon I (1804-15). Second Empire is in reference to the rule of Napoleon III of France (1852-70). New York has been called the Empire State since 1834.

1858 in reference to the emperors of Austria and (after 1870) Germany, from German Kaiser, Bavarian and Austrian spelling variant of of Middle High German keisar, from Old High German keisar "emperor," an early borrowing of Latin cognomen Caesar.

The Germanic peoples seem to have called all Roman emperors "caesar" (compare Old English casere, Old Norse keisari "an emperor"). The word also entered Germanic via Gothic, perhaps from Greek. According to Kluge, one of the earliest Latin loan word in Germanic. The Old English word fell from use after Middle English.