empire (n.)
early 14c., from Old French empire "rule, authority, kingdom, imperial rule," from Latin imperium "rule, command," from imperare "to command," from im- "in" (see in- (2)) + parare "to order, prepare" (see pare).

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). Empire style (especially in reference to a style of dresses with high waistlines) is 1869, from the Second Empire "rule of Napoleon III of France" (1852-70). New York has been called the Empire State since 1834.